Paul was commissioned to proclaim what had previously been a mystery.  During all the Old Testament period, God was preparing His people for the arrival of Jesus (Deuteronomy 18:15).  The laws showed them what pleased and angered God, and the sacrifices provided temporary permission to worship God (Hebrews 10:1-4).  But nothing was permanent or complete until Jesus came to be the Saviour and Redeemer of the world (Hebrews 10:11-14).
 
Paul and the other apostles preached from the Old Testament because it was inspired by the Holy Spirit and contained so much about Jesus Christ (1 Peter 1:10-12).  Before Jesus, the Scriptures were a mystery, but after Jesus died and rose again, the Holy Spirit enabled the apostles to see that Jesus was central to explaining the mystery (1 Corinthians 2:7-10).  Their preaching was full of how Jesus Christ fulfilled the laws and promises of the Old Testament (Acts 3:17-26).

Jesus was never presented as a new religious leader: rather, He was preached as the completion of God's revelation of Himself to humanity, and its sacrificial Saviour (Hebrews 1:1-3).  Augustine of Hippo said, ‘The New is in the Old concealed, the Old is in the New revealed’.  Like Paul, he preached Christ from the Old Testament.
 
Alas, today, too many people like to wallow in the uncertainty of mystery; however, that is an evil temptation when the clarity of Jesus can be known, enjoyed and shared with others.  Others respect the Old Testament for its wise sayings, but have not been alerted to the fact that it speaks of Christ.  2 Corinthians 3:14-16 speaks of such people: "... their minds were made dull, for to this day the same veil remains when the old covenant is read. It has not been removed, because only in Christ is it taken away. Even to this day when Moses is read, a veil covers their hearts. But whenever anyone turns to the Lord, the veil is taken away."  Although they cannot see Christ in the Old Testament, they can see Him when He is shown to be the centre of those Scriptures!  Today is a good day to pray for friends and colleagues who might honour the Old Testament, but do not yet know Jesus.  Pray for an opportunity to show them Christ in the Old Testament (perhaps start with Isaiah chapter 53), because as you explain Christ, He will take away the mystery so that they can see Him clearly.