This letter is primarily written to Gentile Christians who converted from their pagan religions (4:3-4).
The ignorance seems to refer to a time before they knew the revelation of Jesus Christ (1:7) and the ransom from sin (1:18-19).
The passions/desires of their former ignorance are spelled out in 4:3-4.
I would be careful to say that they completely lacked knowledge of God. Paul makes it very clear in Romans 1-2 that we can come to know God through our human reason. Therefore, even the Gentiles mentioned in Romans were without excuse for their sins.
While reason can never come to the full truths of God without the aid of revelation, it can lead us to know some things about God. The ignorance Peter is referring to does not *inevitably *lead to godless conduct, but it certainly doesn’t help.