Why are Christians so judgmental?
This question implies that the questioner has himself made a judgment about Christianity —a negative one. So everyone judges. The question is whose judgment is correct?
There are two probable situations here. The first is that the person asking this question has something in their life over which they simply do not want to be judged. We were given a conscience by God and even without an intellectual understanding of the Bible we all understand by nature and conscience what is right and wrong. But the human condition is that we want autonomy; we want to reject a God that subjects his judgment on us. Plain and simple.
But of course, just because we do not want to be judged does not mean that God does not exist. We will be judged whether we like it or not.
Sometimes people say something like, " My God would not judge." Of course, your statement is correct. Your God indeed would not judge because he does not exist. He is a figment of your imagination. You have created a god in your own image. Such attitude is idolatry, which is a violation of the Second Commandment—a very serious sin.
Actually, it is not the Christian who is judging, but God. A second probable situation here is that the person asking this question just does not appreciate the holiness of God compared to the unworthiness of us humans. They take sin too lightly, failing to understand the depth of all our sin. The Bible says that we have all sinned and fallen short of the glory of God (Romans 3:23) and are deserving of God's wrath (Romans 1:18, 2:5). We have all broken every one of the Ten Commandments in thought, word, or deed. Thus there is an unbreachable gap between God's standard of holiness and our unworthiness. But the good news is that God provided a way to be reconciled to him. As it says in the Bible (Romans 5:8):
But God shows his love for us in that while we were still sinners, Christ died for us.
For more on what the Bible says about the holiness of God, the sinfulness of man, and God's plan of salvation see our Christian Cram Course.