The only book of the Bible that has the specific and stated purpose of preaching the message of eternal life is the Gospel of John. He himself tells us:
Now Jesus did many other miraculous signs in the presence of the disciples, which are not written in this book; but these were written, that you may believe that Jesus is the Christ, the Son of God, and, that, by believing, you may have life in his name.JOHN 20: 30-31
John repeatedly reiterates the only condition to obtain eternal life: to believe in the One whom God has sent (cf. Jn 6:29). In fact, the Greek verb pisteou (to believe) appears 98 times in the Gospel according to John , almost a third of all biblical occurrences.
In the first letter of John the verb appears 7 more times, and the noun also appears once, in one of the most beautiful verses of the epistle:
For whatever is born of God overcomes the world; and this is the victory that has overcome the world—our faith.1 JOHN 5: 4