Albert Barnes Commentary Psalms 99:8

"Thou answeredst them, O Jehovah our God: Thou wast a God that forgavest them, Though thou tookest vengeance of their doings." — Psalms 99:8 (ASV)

Thou answeredst them, O Lord our God - The reference here is to God as “our” God; that is, the language used by those who now worship him is designed to give encouragement in approaching his throne. The God that “we” worship is the same that “they” worshipped; and as he answered them, we may feel assured that he will answer us.

Thou wast a God that forgavest them - They were not perfect; they were sinners; they often offended you, and yet you did answer them and show them mercy.

Though thou tookest vengeance - Though you did manifest your displeasure at their misconduct; though in your judgments you did show that you were displeased with them; nevertheless you did answer them. Sinners as they were, and often as you did show your displeasure at their conduct, yet you did hear their prayers and bless them.

Of their inventions - The Hebrew word denotes work, deed, doing, conduct. It means here what they did—their sins. There is no allusion to any special art or “cunning” in what they did—as if they had “invented” or found out some new form of sin.