"Greet one another with a holy kiss. All the churches of Christ greet you. I appeal to you, brothers, to watch out for those who cause divisions and create obstacles contrary to the doctrine that you have been taught; avoid them. For such persons do not serve our Lord Christ, but their own appetites, and by smooth talk and flattery they deceive the hearts of the naive. For your obedience is known to all, so that I rejoice over you, but I want you to be wise as to what is good and innocent as to what is evil. The God of peace will soon crush Satan under your feet. The grace of our Lord Jesus Christ be with you." Romans 16:16-20 ESV
There are some hard truths sandwiched in that group of verses. I think they are best served up as a sandwich. It starts with love and it ends with grace. The God who deals with obstacles, divisions and evil is a God of peace who promotes peace among the breathren.
The word 'mark' in verse 17 is Skopeo (Strong's G4648) and it speaks of awareness or attention. Not social ostracization or attack. The different doctrine that is problem is the teaching that acts as a snare or creates obstacles that block people from growth.
Flattery is not fellowship but a naive person may not know this is true. The word for evil is Kakos (G2556) and the word translated naive is Akakos (G172). Being naive is being unaware of evil. Being naive is dangerous. Grace teaching is wise teaching that informs of God's goodness and protects a person while keeping them innocent (unmixed) with evil.
Like a good father our peace filled God stomps on the head of the snake.
Love and grace provides vision and ability to scope out the things that slither so that they won't cause division. Naive love not strong in grace teaching can easily fall prey to counterfeit doctrine. An the other hand excessive violence devoid of grace may be applied to the situation resulting in destruction of another kind.
"Watch yourselves, so that you may not lose what we have worked for, but may win a full reward. Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works." 2 John 1:8-11 ESV
This verse is even stronger and a historical frame of reference is important here in order to understand this verse. Teachers traveled from place to place and stayed in believer's houses for periods of time while they taught. The greeting here, coupled with the stay in your home, is not a casual hello or goodbye with a handshake. It is a more formal greeting like the letter a speaker might be sent inviting them to come speak at a graduation. If you invite someone to speak you are giving approval to their teaching because they came at your invitation or you responded to a blanket request made by them.
Pastors who put politicians or philosophers into the pulpit may become partakers of the evil of that powerful soulish person. As John said, 'Watch yourselves.'
I want to be rightly related to these verses as a minister of reconciliation and as a protector of the weak and innocent. Spiritual life is not all smiles and handshakes but it is all about grace, truth and love that makes a difference.