I need a help regarding a grammatical issue in 1 Thessalonians 4:3:
τοῦτο γάρ ἐστιν θέλημα τοῦ θεοῦ ὁ ἁγιασμὸς ὑμῶν ἀπέχεσθαι ὑμᾶς ἀπὸ τῆς
"For this is the will of God, even your sanctification, that ye should abstain from fornication." (KJV)
My question is regarding the phrases "this (τοῦτο)" and "the will of God (θέλημα τοῦ θεοῦ)". Since the phrase "the will of God (θέλημα τοῦ θεοῦ)" serves as an appositive to the phrase "your sanctification (ὁ ἁγιασμὸς ὑμῶν)" which is in the nominative case, I would expect that this phrase also "the will (θέλημα)" would be in the nominative case.
Furthermore, the two phrases are connected by the word "is (ἐστιν)", and I used to think that this word always indicates that the phrases that are connected by it should both be regarded as in the nominative case. However, for some reason, in the highly reputed site BlueLetterBible both of the phrases quoted above are parsed as in the accusative case.
So, can anybody explain to me why is it that way? Or else, is BlueLetterBible wrong in this instance? and not, how should it affect the literal understanding of the verse (since the offered translation seems to hide the phrase above as being in the Accusative and not in the Nominative case)?