Romans 2:1 — Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.
The “therefore” is referring to the fact of the universal knowledge of sin and its consequences as mentioned in 1:32.
Paul is saying that moralists — by which he means “civilized” people such as the Jews, Greeks and Romans — are wrong for judging the heathen (Gentiles) because they are guilty of the same sins inwardly as the heathen are outwardly.
When you recognize sin in others, you prove your own awareness of right and wrong and so are without excuse for your own sins.
Judgest = distinguish between right and wrong and pass and adverse sentence on
Same things — referring in particular to the sins mentioned in 1:29-31.