They stumble at being disobedient to the word, to which also they were appointed. 1Pet 2:8
The passage is to the church -
you yourselves also, as living stones, are being built up as a spiritual house into a holy priesthood...v5
But then, 'they'... They knew the word (λόγῳ (logō), and to which they were appointed.
Who are 'they'? How were they 'appointed'?
Peter states to those whom he has already called ‘living stones’ that ‘to you, therefore, honour’ that is to say the ‘you’ whom he describes as ‘living stones’ have the ‘honour’ of being ‘built up an house spiritual’ and the honour of being ‘an holy priesthood’.
(The translation which states ‘to you therefore he is precious’ is highly debatable. τιμη is 32 times ‘honour’ in the KJV, 9 times ‘price/sum’ and only once ’precious’ - here. The ‘he is’ is in italics, being not in the Greek text.)
‘To you : the honour. But to disobeying : . . . .’
Peter then quotes two texts to the ‘disobeying‘ ; Psalm 118:22 and Isaiah 8:14.
‘To disobeying “the stone which the builders disallowed, the same is made the head of the corner” ; And “a stone of stumbling and a rock of offence”.
The first quotation mentions only, generically, ‘the builders refused’ ; the second goes further in detail and speaks of ‘both the houses of Israel’ and ‘the inhabitants of Jerusalem’.
The ‘he’ who is - to some - a ‘sanctuary’, and - to others - a ‘stone of stumbling and a rock of offence’ is the Lord of hosts, Isaiah 8:13.
Sanctify the LORD of hosts himself; and let him be your fear, and let him be your dread. And he shall be for a sanctuary; but for a stone of stumbling and for a rock of offence to both the houses of Israel, for a gin and for a snare to the inhabitants of Jerusalem. And many among them shall stumble, and fall, and be broken, and be snared, and be taken. - Isaiah 8:13-15
Therefore the stone to which Peter refers is a Divine Person. The Lord of hosts is a stone of stumbling, says Isaiah.
‘They’ . . . ‘stumble at the logos’ . . . . says Peter.
And we know, John 1:1 that ‘God was the logos’ ‘. . . in the beginning’.
And the logos was made flesh, John 1:14.
So the ‘they’ are they that stumble in regard to the One come in flesh.
‘They’ stumble (those that are previously designated, by Isaiah, as ‘both the houses of Israel’ and ‘the inhabitants of Jerusalem) because of their attitude towards the One come in flesh.
‘They’ neither received John the Baptist’s ministry - the baptism of repentance, by the ’messenger of preparation (see Malachi 1:1-3) - nor did they receive Jesus Christ - the ‘messenger of the covenant’ (ibid). Matthew 21:25.
Of Jesus, some of ‘them’ regarded him as the progeny of a supposed relationship involving a Samaritan, John 8:48, and they regarded him as having a demon. To which slanderous and outrageous lies, Jesus answered nothing.
‘They’ strove often with Jesus as to his origin. They sought to kill him when he spoke of his Father. They hated to hear of the relationship of Father and Son. ‘They’ picked up stones to stone him to death, John 8:59 and 10:31.
In the end, ‘they’ crucified the Lord of glory, 1 Corinthians 10:8.
‘They’ says Peter were ‘disobedient’. They did not obey Moses. Had they hearkened to Moses, Jesus pointed out, they would have believed on himself, John 5:46.
They obeyed neither Moses nor the prophets who prophesied of Messiah, nor did they obey John the Baptist.
Then they fell flat on their face when confronted with Jesus of Nazareth, born of a virgin, the Monogenes and the Prototokos.
Because of disobedient unbelief.
I am covering only the first question.
I think the second - the matter of ‘how’ they were appointed - should be a separate question, myself.
The Who are the Pharisees, scribes and elders who were against Jesus. These were considered the builders at the time Jesus was alive. These are the builders of the temple so to speak and they rejected the very corner stone of the whole foundation which was Jesus Christ.
Jesus is giving a parable as He's actually talking about them in the parable of the vineyard.
6Therefore, having yet one son, most dear to him, he also sent him unto them last of all, saying: They will reverence my son. 7But the husbandmen said one to another: This is the heir. Come let us kill him and the inheritance shall be ours. 8And laying hold on him, they killed him and cast him out of the vineyard. 9What therefore will the lord of the vineyard do? He will come and destroy those husbandmen and will give the vineyard to others. 10And have you not read this scripture, The stone which the builders rejected, the same is made the head of the corner: 11By the Lord has this been done, and it is wonderful in our eyes. 12And they sought to lay hands on him: but they feared the people. For they knew that he spoke this parable to them. Mark 12:6-13
They are also mentioned in Isaiah 28:12-16 The leaders of Israel had burdened the people with "do this and do that" and so many laws that the people were bent over. These were the rulers in Jerusalem. So what does He do? God himself sets up a corner stone in the temple that causes these rulers to stumble and fall over. Those who trust in that corner stone will be at peace.
This is the place of repose”— but they would not listen. 13So then, the word of the Lord to them will become: Do this, do that, a rule for this, a rule for that; a little here, a little there— so that as they go they will fall backward; they will be injured and snared and captured. 14Therefore hear the word of the Lord, you scoffers who rule this people in Jerusalem. 15You boast, “We have entered into a covenant with death, with the realm of the dead we have made an agreement. When an overwhelming scourge sweeps by, it cannot touch us, for we have made a lie our refuge and falsehood b our hiding place.” 16So this is what the Sovereign Lord says: “See, I lay a stone in Zion, a tested stone, a precious cornerstone for a sure foundation; the one who relies on it will never be stricken with panic.
Yet even in this, these rulers were appointed and placed there by God. It was necessary for them to be a part of God's plan in crucifying the Lord Jesus.
Man has no part in building the temple for His dwelling. His Son will build it being the chief corner stone and everyone that He has chosen becomes a living Stone that God places where He sees fit.
They were appointed and placed there for this very purpose by God.
The stones, the chosen ones are new generation that were chosen at that time to be a part of the building, temple.
4As you come to Him, the living stone, rejected by men but chosen and precious in God’s sight, 5you also, like living stones, are being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ. 6For it stands in Scripture: “See, I lay in Zion a stone, a chosen and precious cornerstone; and the one who believes in Him will never be put to shame.”
