For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
I'm not 100% sure, but weren't they teaching the Old Testament? Also, possibly the miracles & healings that they witnessed Jesus perform?
No, Paul wasn't teaching the same thing necessarily. Paul taught to the Gentiles, & the other disciples taught to the Jews.
JD, very accurate! Bridget, you are correct! To continue:
As a result of Israel's blaspheming of the Spirit through the stoning death of Stephen, God offered salvation to the Gentiles as a free gift, without having to first go through Israel to receive it. This was to provoke Israel to jealousy (Rom 11:11).
The 'mystery' of how things work now (Eph 3:2), is in the gospel of God's grace, given only to Paul from Christ ascended (Gal 1:12). It is from this gospel of Christ crucified (never addressed by the twelve) that believers obtain salvation by faith alone in Jesus Christ (Gal 2:16), and become 'fellowheirs' with Christ and partakers in His body (Eph 3:6).
Jesus while living never told the twelve to stop following Mosaic law. What He did tell them was to "follow me". He was leading Israel to their promised earthly kingdom of Heaven. They were to reign with Christ upon receiving it, and still one day will.
Christ from Heaven however tells Paul that Paul has been commissioned to fulfill the word of God (Col 1:25), and that we are to now follow him even as he follows Christ (1 Cor 4:16, 1 Cor 11:1, Phil 3:17). The life of Jesus Christ showed us God in the flesh, but we are no longer capable of following Jesus in the flesh (2 Cor 5:16), so we follow Christ from Heaven, spiritually now. Paul's epistles detail this transition (Phil 3:20).
Israel has an earthly kingdom that awaits her, but the church, the body of Christ, seeks spiritual blessings in Heavenly places (Eph 1:3).