Is it a sin to call someone a fool?

This question must be answered Yes and No. While it is correct that Jesus used the word fool and said that if you call someone that you will be cast into hell, we cannot just take that verse out of the rest of the Bible and say it is a sin to call someone a fool. In Matthew 5:22 Jesus stated, “Whoever says, ‘You fool’ shall be in danger of hell fire.” However, if you look at what else Jesus said when He said this it is clear that he is talking about something bigger than using the word fool. He is speaking of anger and bitterness that we harbor towards other people. This is a much deeper issue than using the word fool. It is, as it almost always is with Jesus, a heart and sin issue, not a verbal word usage issue. Besides if it is a sin to simply call someone a fool then God himself would be a sinner as the verses listed below all contain God calling people fools.
So who does God call a fool? The short answer is that God calls a person a fool who does acknowledge the power of God in this world and who lives their life in rebellion to God. That person is a sinner and a fool in the eyes of God.
In answer to the question then, I think we should discretion when calling people fools and not rush to such a powerful word when talking to people. However, the Bible is clear as to who the fools of this world are. Those that do not live for Jesus and submit to his authority on this earth are living and building lives on shifting sand, and there is no greater fool than a man who builds his house on a foundation that will fall one day.

Psalm 14:1 and Psalm 53:1 – Those who don’t believe in God are called fools

Proverbs and Ecclesiastes use fool repeatedly

Luke 12:20 God calls the man who does not honor Him with his money a fool.

1 Cor. 15:36 – Paul calls those who deny the power of God to raise Jesus from the dead fools.