I would suggest Matthew,especially. The Sermon on the Mound,especially the verses about hate being the equivalent of murder and lust being equal to adultery.the first step in Christianity is escapling The Fool’s Paradise of thinking “i’m Not perfect,but I’m doing the best I can—God will accept me! “ If the Holy Spirit convicts them as a lost sinner,bankrupt in their “worthiness” before God.....then ,perhaps they are ready for Romans where they can discover Justification by Faith and 1Cor:15.....Paul’s Gospel thAt saves