What is enemy of the Cross doctrine?.... Never heard of it....
It comes from Philippians 3:18
"For, as I have often told you before and now tell you again even with tears, many live as enemies of the cross of Christ."
Paul is referring to the religion of the Judaizers, which places confidence in performing the rituals of the law, particularly ritual rabbinical circumcision, rather than faith in the work of Christ on the cross to be at peace with God. Any work a person trusts in to bridge the gap between them and God makes them an enemy of the way of the cross to peace with God.
I thought that was sin...
It is. It is a sin to not trust in Christ but instead trust in works of the law, or any work to be at peace with God in salvation. But not only that, trusting in anything but the work of Christ for righteousness will invariably be marked by that person still being in bondage to their life of sin. They will just think they are living right and at peace with God.
That's what trusting in 'religious' works does to a person. It gives them a false sense of satisfaction that they are living right and pleasing to God. When, actually, it is only a change in your character that indicates you are in a relationship that is right and pleasing to God.
People need saved... How do we bring them to Messiah?
Help them see the futility of anything but the forgiveness of God in Jesus to bring them to peace with God. No work except the work of Jesus on the cross can repair the divide between fallen man and God.
When a person comes to the end of themselves and the only thing left for them to do is to lift their hands up in surrender and ask for mercy, that is when they can be brought to Messiah.
‘God, be merciful to me, the sinner!’-Luke 18:13
