Grammar and Meaning in Context of EIUS

Question

I have a question regarding the translation of the Gregorian chant OS JUSTI, which Wikipedia claims to be used as gradual of the Commune Doctorum, and as introit I and gradual II of the Commune Confessoris non Pontificis. (I have the text from a choral piece of Anton Bruckner and did not check Bruckner's source.) The text reads:

OS JUSTI MEDITABITUR SAPIENTIAM
ET LINGUA EIUS LOQUETUR JUDICIUM
LEX DEI EIUS IN CORDE IPSIUS
ET NON SUPPLANTABUNTUR GRESSUS EIUS

I wonder about the of grammar of EIUS in the different verses: If I see it correctly, this is Genitive Singular of IS. And I cannot bring this into accordance with the meaning of the verses I assume:

I read

ET LINGUA EIUS LOQUETUR JUDICIUM

as something like And his tongue speaks right, so EIUS means his. But how is this? Why is not the more direct (?) SUUS being used here?

The analogue problems occurs at

LEX DEI EIUS IN CORDE IPSIUS

Here I am not sure if this must be read as The law of his god is in his heart or simply as The law of god is in his heart, and of course also at

ET NON SUPPLANTABUNTUR GRESSUS EIUS

which I read as And his pace will not be made stumble.

The accepted answer of What is the difference between suus and eius? states, that EIUS, in contrary to SUUS is usually not referring to the subject of the main clause. But I don't think that this rule does apply here, becaus JUSTUS is the only person present in the whole text.

Answer 1

The rule does apply -- it's a syntactic rule, not a semantic one. Since the subjects are lingua etc. rather than justus, the possessives are not reflexive, so eius is used rather than suus.

For lex dei eius, this does mean the law of his god -- there's nothing else eius could be doing here. Looking at the Hebrew source confirms this -- the text is taken from Psalm 37:30-31, which does indeed say תּוֹרַת אֱלֹהָיו בְּלִבּוֹ The law of his god is in his heart.