I write these things 1 John 5:13

1. I write these things to you who believe in the name of the Son of God so that you may know you have eternal life.

2. For my Father's will is that everyone who looks to the Son and believes in Him shall have eternal life. I tell you the truth, he who believes has everlasting life (John 6:47).

See also John 20:31

But these (miracles performed but not recorded in this book) are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name.