Romans 2:2-3: "And we know that the judgment of God rightly falls upon those who practice such things. And do you suppose this, O man, when you pass judgment upon those who practice such things and who do the same yourself, that you will escape the judgment of God?"
The first part refers to the gross sins mentioned in the last half of Romans 1. But the second part refers to those who have not committed those kind of sins, but who sit in judgment on those who do, as from a moralistic "I am better than you" way of thinking.
Macarthur remarks, on p. 115 of his Romans 1-8 commentary:
"Men are so used to God's blessings and mercy that they take them for granted, not realizing that they receive those things purely because of God's long-suffering and grace. God would be perfectly just to blot out any person or all persons. But human nature trades on God's grace, believing that everything will work out all right in the end because God is too good and merciful to send anyone to hell. As someone astutely observed, 'There is some kind of a still little voice in everybody that constantly convinces them that in the end it's going to be OK.' That little voice speaks from a person's fallen nature, which constantly seeks to justify itself.
"Paul sternly warns against such false confidence. Although he was conscious of no specific unconfessed sin in his life, even he knew better than to rely on his imperfect human judgment, declaring, 'I am not by this acquitted' but the one who examines me is the Lord' (I Cor. 4:3-4). He knew that every person's discernment is hopelessly distorted and cannot make a proper evaluation even of his own spiritual health, much less that of someone else."