This seems so obvious that it need not be said, but God includes it in His Word because Christians within the church do not hold in check some of the very things that cause so much disunity in this world. The apostle entreats us to lay aside the causes of contention so we can live in harmony.
Sometimes we do not understand how competitive human nature is. It is proud. It feels it has to win, be vindicated, and if possible, elevated over others. These attitudes do not make peace. Rather than pursuing the things that cause contention, Paul says, pursue the things that cause peace. It is a Christian's responsibility, part of his vocation. Emphasizing the positive is an incomplete, but nonetheless fairly accurate, description of what can be done.
Solomon writes in Proverbs 13:10, "By pride comes only contention, but with the well-advised is wisdom." Contention divides. Much of the strife and disunity in the church is promoted by those who seem bent on "majoring in the minors." This is the overall subject of Romans 14. Church members were becoming "bent out of shape" over things that irritated them but had little or nothing to do with salvation. They blew these irritants out of proportion to their real importance, creating disruption in the congregation.
Essentially, Paul tells these people to change their focus, to turn the direction of their thinking, because we agree on far more that is of real, major importance to salvation than what we disagree on. If we will cooperate on these major things rather than on private ends and prejudices, peace and unity will tend to emerge rather than strife and disunity. Paul further admonishes the irritated members to have faith in God's power to change the other: "Who are you to judge another's servant? To his own master he stands or falls. Indeed, he will be made to stand, for God is able to make him stand" (Romans 14:4).
Why can we not cultivate a spirit of peace by striving for holiness? Holiness is a major issue leading to preparation for God's Kingdom and salvation. Peace is one of its fruits. Why can we not show love for the brethren and strive to do good for them "as we have opportunity, . . . especially to those who are of the household of faith" (Galatians 6:10)? Why can we not spend more serious time studying God's Word, getting to know Him? These admirable pursuits are humbling and serving. They produce peace and put other, less important matters into a proper perspective and priority. If pursued sincerely, they keep the "minors" right where they belong because they tend to erode one's pride.