Can you explain the meaning of the phrase "dead in the trespasses and sins" in Ephesians 2:1? Is this an idiom? Or is it part of an overarching theology in New Testament, the Epistles, or whole bible? or is it only used here?
And you were dead in the trespasses and sins
In Ephesians 2:1, "dead in trespasses and sin" is a reference to someone that does not have faith in Christ. In Luke 15:20-24, the phrase is a little different but used in a similar context, and it also shows how remorse is needed for salvation.
Regarding your first example (Ephesians 2:1):
In the very first sentence of Ephesians (1:1), Paul states who he is talking to.
"Paul, an apostle of Jesus Christ by the will of God, to the saints which are at Ephesus, and to the faithful in Christ Jesus. (Ephesians 1:1, KJV)
Paul then makes it clear what the faithful will receive...
"Grace... peace... spiritual blessings... holy and without blame... accepted... redemption... forgiveness of sins... wisdom and prudence... an inheritance." (Ephesians 1:2-11, KJV)
...and again, why they will receive it...
"That we should be to the praise of his glory that first trusted in Christ... In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy spirit of promise. " (Ephesians 1:12-13, KJV)
Ephesians 1:14-23 continues in this light and it is the backdrop for Ephesians 2:1-2, where Paul makes it clear to the church of Ephesus what they were like before they had this faith.
"And you were dead in the trespasses and sins in which you once walked following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience." (Ephesians 2:1-2, ESV)
Regarding your second example (Luke 15:20-24):
In Luke 15:1-10, Christ is relaying his Parable of the Lost Sheep. What shepherd, with a flock of sheep, isn't going to go after the one that goes astray? (Luke 15:4) The disciples can relate to this analogy. Here, Christ compares himself to a shepherd and his disciples to a flock of sheep. He explains how the one sheep that gets away is worth everything at that moment and how it is cause for celebration when the shepherd finds that sheep. What's interesting is that this is an analogy. Christ could have easily made a point about a shepherd reuniting with his sheep. He adds remorse.
"I say unto you that likewise joy will be unto heaven over one sinner that repenteth, more than over ninety and nine just persons, which need no repentance." (Luke 15:7, KJV)
"Likewise I say unto you, there is joy in the presence of the angels of God over one sinner that repenteth." (Luke 15:10)
In Luke 15:11-24, after the Parable of the Lost Sheep, Christ uses the same analogy. Here, the father loses his son; the son feels pain, away from his father; the son has remorse and reunites with his father. Here, Christ is talking about a person disconnected from YHWH that connects to YHWH, through remorse. When the father says, "my son was dead and is alive again; he was lost and is found," that's not only congruent with the Parable of the Lost Sheep -it's congruent with Ephesians 2:1, where the disbelievers were "dead in the trespasses and sins" until they had faith.
In Scripture, Being "Dead in Sin" is seen as being separated from the covenant promises of God - because of Transgression :
The Parable of the Prodigal Son illustrates the loss of covenant relationship, and inheritance - through transgression. (Also, see Covenant Theology):
NASB, Luke 15:21,30-32 21 And the son said to him, ‘Father, I have sinned against heaven and in your sight; I am no longer worthy to be called your son.’ 30 but when this son of yours came, who has devoured your wealth [his inheritance] ...’ 31 And he said to him, ‘Son, ... we had to celebrate and rejoice, for this brother of yours was dead and has begun to live, and was lost and has been found.’”
Hebrew and Greek Scriptures Consistently Associate Transgression with Loss of God's Blessings :
NASB, Ephesians 4:17-18 : So this I say, ... walk no longer just as the Gentiles also walk ... 18 being darkened in their understanding, excluded from the life of God ... because of the hardness of their heart;
NASB, Ezekiel 18:31 - Cast away from you all your transgressions which you have committed and make yourselves a new heart and a new spirit! For why will you die, O house of Israel? 32 For I have no pleasure in the death of anyone who dies,” declares the Lord God. “Therefore, repent and live.”
Note: All throughout Hebrew Scripture "כָּרַת" (Kareth, Wikipedia Link) is the term meaning to "Cut off". This meant to be separated from the covenant and promises of God because of disobedience. ("Cut off" word search).
In the New Testament, there are only two states: Alive in Covenant - through trust in Jesus; or outside of covenant, because of transgression - even Israel.
Ephesians, (beginning in Chapter 1), consistently speaks of heavenly blessings, (Ephesians 1:3), as a Spiritual Inheritance :
ESV, Ephesians 2:1 - And you were dead in the trespasses and sins ... 5 ... made us alive together ... in the heavenly places in Christ Jesus ... Eph. 3:6 - to be specific, that the Gentiles are fellow heirs and fellow members of the body ...
Note: "becoming alive" when "together with Christ" - through trust [faith], (Eph 2:21; 22; together with the body : 4:16; 4:17).
In Ephesians, the context repeatedly juxtaposes "reconciliation to" and "separation from" God, (as strangers, aliens, etc); to be specific ...
NASB, Ephesians 3:6 - to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel, ...
Ephesians 2:11 - Therefore, remember that formerly you who are Gentiles ... 12 remember that at that time you were separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world. 13 But now in Christ Jesus you who once were far away have been brought near [vs. being "cut off"] by the blood of Christ.
Required for even Israel too, (those obligated under the law) :
NASB, Hebrews 9:15 - For this reason [Jesus] is the mediator of a new covenant, so that, since a death has taken place for the redemption of the transgressions that were committed under the first covenant, those who have been called may receive the promise of the eternal inheritance.
Obedience and Life in Hebrew Scripture :
NASB, Isaiah 55:3 - “Incline your ear and come to Me. Listen, that you may live; And I will make an everlasting covenant with you, According to the faithful mercies shown to David.
[Heb 9:26 ESV] 26 for then he would have had to suffer repeatedly since the foundation of the world. But as it is, he has appeared once for all at the end of the ages to put away sin by the sacrifice of himself.
– Ruminator Dec 02 '18 at 12:43Death in sin is spiritual death.
You can find an Orthodox Christian exegesis of this expression in Theophylact's commentary on the Ephesians passage (he was a Byzantine Greek, writing in Greek):
Paul stated above (ch. 1:19-20) that God the Father displayed the same power and divine energy towards us as He had displayed in Christ. He also affirmed that the Father raised Christ from the dead and seated Him far above all creation, visible and spiritual. Now, addressing us, the Gentiles, he declares that you, who were dead - not from the death in the body that began with Adam's transgressions, but from the spiritual death of sin that we brought upon ourselves - God has raised and quickened. Thus, with the same power by which the Father raised Christ, in His human nature, from bodily death, so He has raised us from the spiritual death of sin. Indeed, God performed a greater miracle in changing the disposition of the soul than in raising the dead.
The Explanation of the Epistle of Saint Paul to the Ephesians, with additional commentary by Nicodemus of Mt. Athos (tr. by Chrysostom Press, 2013), p.30
The Greek word for "trespass" is παράπτωμα (paraptōma). Another passage in Scripture relating "trepasses" (or "offenses") with death is Romans 5:15 -
For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
Both verses refer to the disruption of man's relation to God. The former (Ephesians) refers to the continuing disruption due to our continued commission of offenses, whereas the latter (Romans) refers to the ancestral sin of mankind's fall in the garden (see, e.g. Wisdom 10:1).
You have to start with Adam in the Garden! What happened to Adam when he sinned? His Spirit Died His body or flesh lived on!
That is why man must be born of the Water and the Spirit to enter Heaven John 3:5 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
All people born after Adam were born without the Spirit that Communicated with God!
Example John 9:31 King James Version
31 Now we know that God heareth not sinners: but if any man be a worshipper of God, and doeth his will, him he heareth.
Verses that prove the Spirit has to be brought back to life!
Psa_119:50 This is my comfort in my affliction: for thy word (((hath quickened me))).
Psa_119:93 I will never forget thy precepts: for with them thou (((hast quickened me))).
Eph 2:1 And you hath he quickened, (((who were dead in trespasses and sins)));
Col 2:13 And you, (((being dead))) in your sins and the uncircumcision of your flesh, (((hath he quickened together with him)), having forgiven you all trespasses;
Eph 2:5 (((Even when we were dead in sins))), hath (((quickened us))) together with Christ, (by grace ye are saved;)
1Pe_3:18 For Christ also hath once (((suffered for sins))), (((the just for the unjust))), that he might (((bring us to God))), (((being put to death))) in the flesh, (((but quickened by the Spirit))):
All those verses talk about being dead including Jesus, (((Being Dead in the Flesh))) but were Quickened by the Spirit! (((Quickened means bring to life!)))
Websters Definition of quicken transitive verb 1 : to make alive : REVIVE
Now does the verse below make more sense to you?
King James Version Romans 8:11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
That Spirit was not Alive in the Baby when it was born! If it had been the Baby would never have to be saved when it reaches the age of accountability! The ONLY WAY MAN RECEIVES THAT SPIRIT IS TO BELIEVE! John 6:29 Why is it so hard for people to believe that?
James 2:26 is commonly mistranslated as "the spirit is dead" instead of the correct "the body apart from the breath is dead":
[Jas 2:26 ESV] 26 For as the body apart from the spirit is dead, so also faith apart from works is dead.
Apart from the breath of life, the body is just a piece of dirt:
[Gen 2:7 ESV] 7 then the LORD God formed the man of dust from the ground and breathed into his nostrils the breath of life, and the man became a living creature.
In the same way those devoid of God's holy spirit are spiritually dead. Paul calls all this "the principle of the breath of life":
[Rom 8:2 ESV] 2 For the law of the Spirit of life has set you free in Christ Jesus from the law of sin and death.
IE: "dead" in the Ephesians context is to be "in the flesh" and without the spirit of God. The inverse is also true, that if you have the spirit then you have life:
[Rom 8:10 ESV] 10 But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness.
This principle of the "breath of life" is evident throughout the scriptures:
[Eze 37:9 ESV] 9 Then he said to me, "Prophesy to the breath; prophesy, son of man, and say to the breath, Thus says the Lord GOD: Come from the four winds, O breath, and breathe on these slain, that they may live."
[Job 33:4 ESV] 4 The Spirit of God has made me, and the breath of the Almighty gives me life.
[Isa 38:16 ESV] 16 O Lord, by these things men live, and in all these is the life of my spirit. Oh restore me to health and make me live!
[Rom 8:10-11 ESV] 10 But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness. 11 If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.
[2Co 3:6 ESV] 6 who has made us sufficient to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life.
[Psa 104:27-30 ESV] 27 These all look to you, to give them their food in due season. 28 When you give it to them, they gather it up; when you open your hand, they are filled with good things. 29 When you hide your face, they are dismayed; when you take away their breath, they die and return to their dust. 30 When you send forth your Spirit, they are created, and you renew the face of the ground.
[Jhn 6:63 ESV] 63 It is the Spirit who gives life; the flesh is no help at all. The words that I have spoken to you are spirit and life.
So to be dead in sin is to be void of the holy spirit which is the breath of spiritual life.
Update
Since posting the above I have observed that Paul uses 2 Samuel 9 as to provide his framework to discuss God's kindness to the gentiles (along with the Jews). In that chapter Mephibosheth refers to himself as a "dead dog". "Dog" often means "gentile" and "dead" in that context referred to his being a potential rival to David and thus doomed:
https://www.quora.com/Can-you-explain-the-bible-book-of-Ephesians/answer/Bill-Ross-22
