1. Christ and evangelism are the only things that are not in vain
    in this world. (Ecc 1:2)
    Dan 12:3, 1 Thess 2:19-20, Isa 40:8, Mt 24:35, Mk 13:31, 1 Pt 1:25, Rev 1:17-18, Rev 2:8, Rev 22:12-13

In the Old Testament, the son of David confessed that all things in the world were in vain. (Ec 1:2)

Ec 1:2 Vanity of vanities, saith the Preacher, vanity of vanities; all is vanity.

In the Old Testament, Daniel said that those who turn many to righteousness will shine like a star forever and ever. (Dan 12:3)

Dan 12:3 And they that be wise shall shine as the brightness of the firmament; and they that turn many to righteousness as the stars for ever and ever.

In the Old Testament, Isaiah confessed that only the Word of God is eternal. (Isa 40:8)

Isa 40:8 The grass withereth, the flower fadeth: but the word of our God shall stand for ever.

Only the words of Jesus are eternal. The eternal word in the Old Testament is the word of the gospel that Jesus is the Christ. (Matthew 24:35, Mark 13:31, 1 Peter 1:25)

Matthew 24:35 Heaven and earth shall pass away, but my words shall not pass away.

Mark 13:31 Heaven and earth shall pass away: but my words shall not pass away.

1 Peter 1:25 But the word of the Lord endureth for ever. And this is the word which by the gospel is preached unto you.

Jesus, the Christ, is the first and the last and the eternal. (Rev 1:17-18, Rev 2:8, Rev 22:12-13)

Rev 1:17 And when I saw him, I fell at his feet as dead. And he laid his right hand upon me, saying unto me, Fear not; I am the first and the last:
18 I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.

Rev 2:8 And unto the angel of the church in Smyrna write; These things saith the first and the last, which was dead, and is alive;

Rev 22:12 And, behold, I come quickly; and my reward is with me, to give every man according as his work shall be.
13 I am Alpha and Omega, the beginning and the end, the first and the last.

Those to whom we have preached the gospel become our glory and joy forever. (1 Thessalonians 2:19-20)

1 Thessalonians 2:19 For what is our hope, or joy, or crown of rejoicing? Are not even ye in the presence of our Lord Jesus Christ at his coming?
20 For ye are our glory and joy.