Jn 5:39, Jn 6:29, Jn 17:3

In the Old Testament, the son of David, the evangelist, said that the duty of man is to fear God and keep the word of God. (Ecclesiastes 12:13)

Ecclesiastes 12:13 Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man.

Jesus revealed that the Old Testament testifies of Christ and that Christ is Himself. (John 5:39)

John 5:39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.

It is God's work and eternal life to believe that Jesus is the Christ, the one sent by God. (John 6:29, John 17:3)

John 6:29 Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent.

John 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.