A.T. Robertson Commentary John 15:22

"If I had not come and spoken unto them, they had not had sin: but now they have no excuse for their sin." — John 15:22 (ASV)

They had not had sin (αμαρτιαν ουκ ειχοσαν). Conclusion of condition of second class without αν because context makes it clear (νυν δε) without it (Robertson, Grammar, p. 1013). The imperfect active indicative with -οσαν instead of -ον (also in verse 24) as common in the LXX, and occurs in the papyri and the inscriptions and the Boeotian dialect.

Excuse (προφασιν). Old word (1 Thessalonians 2:5) either from προφαινω, to show forth, or προφημ, to speak forth. Mere pretence, in John only here and verse 24.