1 John 5:10

Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son.

(v10, NIV)

If we are a man (or woman) then our testimony is that of a man. John was just talking about how God's testimony is greater than the testimony of men. However, if we believe in his son, then God's testimony is in our heart. Therefore, the testimony in us that we can speak is the testimony of God and not that of men. With God's testimony in us, we become those who testify of what God testifies. We become proclaimers of the truth of his son. No longer is our testimony that of men, but it is the testimony of God, which is greater than man's testimony.

If we claim we have not sinned, we make him out to be a liar and his word has no place in our lives.

(1 John 1:10, NIV)

Looking back at chapter 1, we see that claiming to be without sin makes him to be a liar. Another way in which we make God to be a liar is if we do not believe God. This is fairly simple and easy to see how it makes sense. If we don't believe what someone says, we believe that what they are saying is not true. Therefore, we believe what they are saying to be a lie. Someone who tells lies is a liar, so we are calling that person a liar. In the same way, if we disbelieve what God says, we are calling him a liar. If you do not believe in God's testimony then you are calling him a liar. God testifies of his son, and if you do not believe in his son, you are calling God a liar. If you want to call God a liar, go ahead. But I wouldn't advise it; it is not a wise decision.