"Who is it that overcomes the world? Only he who believes that Jesus is the Son of God." NIV translation
John repeats that it is not only believing that Jesus was a man or a prophet but believing that He is and was the Son of God. Many people believe that Jesus existed and was a good man or a prophet but that is not a saving faith. For example, Islam teaches that Jesus was a prophet but strongly denies Him as the Son of God and so Muslims (followers of Islam) have not overcome the world. In fact, as we saw in chapter four, they are controlled by the spirit of the antichrist.
"This is the one who came by water and blood - Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. For there are three that testify: the Spirit, the water and the blood; and the three are in agreement." NIV translation
Throughout the Bible, water is a symbol of the Word of God. This passage shows us the relationship between the word, the Spirit, and the sacrifice of Jesus Christ. The Living Word (Jesus Christ) came and paid for our sins with His blood allowing us a right relationship with God. His word is applied to our lives through the power of the Holy Spirit.
The number three is associated with the earthly display of God's will and we see through this passage that God's will is for us to accept the sacrifice of His Son and to walk in His word through the power of the Spirit.
"We accept man's testimony, but God's testimony is greater because it is the testimony of God, which he has given about his Son. 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." NIV translation
We believe many things that people tell us today without even thinking about it. At the same time, many reject the words of God concerning His Son. God's testimony about Jesus was not with words only but with action demonstrated by His resurrection. Anyone who does not believe that Jesus is the Son of God, attempts to call God a liar even as evidence was presented by that resurrection.