First, everyone doesn't "know" the Church's teachings on such things. Which "marriages" require an annulment is even confusing to me sometimes, and I am supposed to know this stuff... I doubt many people pick up the Code of Canon Law and read the chapter on marriages at the beginning or the end of such a relationship.
Also, regarding "unknown sins", I think you will find support for the concept. The OT especially has verses that ask God to forgive us of sins we may have commited but were unaware of them. I do not believe that sins commited ignorantly are as dangerous to the soul as the willful sins. Hebrews 10, for example, discusses willful sins, not sins in general commited out of ignorance. It is the rejection of God that locks us out of heaven, not following a rule book we may not be aware of.
I am painfully aware of the fact that when I judge others, I condemn myself. Still I will take a stand on adultery.
Well, I appreciate that stand. However, being in a position to see people on a real life basis in these situations (a person who married someone who has not had their last relationship annuled), I have a harder time "condemning" them. They truly were acting in good will when they married civilly. They love their wife/husband and have a good relationship. It is difficult for me to "condemn" someone because their spouse's previous relationship was not officially annuled by the Church. How am I as an RCIA instructor supposed to slam the door in people's faces because of that? It is very hard, to judge others based on their ignorance of Canon Law, when they are actively seeking God and desiring to become Catholic. These things are delicate subjects, and as such, I have a harder time making such decisions that are not so black and white. Naturally, we do not accuse them of adultery!
Considering this verse, what do you think will happen to a person that accuses someone of losing his salvation?
I think Paul is refering to the Gentile who has a general tendency to sin, not a good person who ignorantly breaks ecclesiastical rules. I think we can determine the wicked from the decent person. The context of the verse (Romans 2:1) is written towards the Jew/Christian who is proud of their religiousity, while they do the very same things that the pagans do.
Regards