God's grace brings salvation and teaches us to say no to unrighteousness and contrasts the divisiveness of man's way (hate, contention, ...) with the unity of God's way, that is love and peace.
Because of God's great sacrifice, we have the hope of eternal salvation. This act of Grace on God's part (saving us when we do not deserve to be saved) shows us/gives us an example of how to behave. We, too, should love those around us, even if they do not deserve it. And as such, we should support and build up, not gossip and criticize uselessly; pointing people to the correct path with love and compassion, not point out faults in spite.
Here is a conundrum. We are urged to let no man despise us, but at the same time to show true humility to all men. How then can we do this? By our actions. When someone despises us because of our "weakness", we need to ask God to help us show them that our weakness is God's strength.
Our helping the poor, the needy, the loveless and unloved brings us in line with God's will. Miracles and joy and peace will come from it. While those that despise us and slander us will only bring approbation upon themselves for belittling good (as all of God's ways are). Our humility and respect and love and nature of servanthood that we copy from God will, we pray, make men love God. When people are not nice and don't appreciate our help, remember, we once walked in the ways of ungodliness, "..being hated and hating one another."
Finally, we are to avoid foolish controversies concerning the law, and who is better than whom. They do not profit us, but show the world that we do not present a united body, being of one mind, the mind of Christ.
There are important things that we should argue about, Christ Himself told us that there would be heresys, as did Paul. But, on the small points, we should agree to disagree and avoid the contention. Do not argue with one who wants to argue . Warn them twice and then ignore them. Without someone to argue with they will just be useless and look silly. Be sure that this man has bigger problems that he is trying to avoid by focusing on the unimportant. If we do not allow him to focus on the unimportant, perhaps he will be confronted with his major problems and do something about them.
But this is what we all do. We focus on the unimportant. We have to watch a certain program on TV. We even have VCR's and TIVO so we can watch it later if we have to miss it. But, do we take the time to pray and read the Bible regularly? Is this not much more important, in eternity, than anything of the world?