When Jesus outlined the steps in church discipline in Matthew 18:15-18, though it was still the period when the Old Law of Moses was still in effect, Jesus was communicating an important teaching for the New Covenant. Jesus lived under the Old Law, Gal 4:4,5. The Old Law was done away when Christ and Faith came, Gal 3:24-26. Though Jesus lived under the Old Mosaic Law, it does not mean that He didn’t exemplify and communicate the life of the New Covenant that would go into effect at His death on the cross, Heb 9;15-17. The great commission included baptism and teaching the disciples to observe all which Jesus commanded, Mt 28:19,20. So the Old Law which Jesus lived under was in force until the Cross. The tell of the life Jesus lived under Moses’ Law. But also, the New Law of Christ was being communicated throughout the Gospels. Matthew, Mark, Luke, and John are called Gospels because they tell the good news of Jesus. The Good News is what the Church preaches and it is what sinners obey, Rom. 10:16; 2 Thess 1:7-9; 1 Pt 4:16,17. The teachings of Jesus Christ found in the four Gospels, including the teaching by Christ on marriage in Matthew 19 and Matthew 5, are part of the teachings for the church of Christ.