After baptism, it is essential to continue in obedience of all the things we have been commanded by Christ to do. Of course, we all fail daily in matters of walk and conduct. We confess our failures, pray for forgiveness and strength to overcome our weaknesses, and go on.
When it comes to obeying the truth, it is a command by Christ that we walk with those who are walking in the truth, and that we refuse to walk with those who are walking in error. If one defends his sin, or if he strays from the truth of the Gospel, we cannot escape our own responsibility apart from separating from those in error.
The word "partaker" means fellowship which also means partnership. If we fellowship those whose beliefs or actions are in error, we ourselves become guilty for fellowshiping such, and become partners in the error.
The commandment was to deliver him back to Satan, that is, back to the world, and out of the ecclesia till he understood the truth as it is in Jesus. Because "a little leaven leaveneth the whole lump." That is, the error, if uncontrolled will spread in the body until the whole truth is lost.
We also must take great care that we never withdraw from those who are walking in the Truth of the Gospel, confessing their sins. Man is so prone to crotchets, that a whole class of men had been developed in Jesus' lifetime, called the Pharisees, who were guilty of "teaching for doctrines the commandments of men" Mat. 15:9. We must not join this class in going beyond what Jesus himself required of our brethren, placing obstacles in their path that Jesus did not place through rules based on our personal prejudices. And, we must also never walk with those who are walking in error, leavening the truth with error and wickedness. Such a balance requires careful study and discernment!