"God is faithful, who has called you into fellowship with his Son, Jesus Christ our Lord." — 1 Corinthians 1:9
Paul diligently taught that God is faithful, calling people into fellowship with his Son. And for the sake of being faithful witnesses for Christ to the world, it's essential that all who are called into this fellowship live in unity and harmony.
But things weren't going that way in the church at Corinth. And Paul minced no words in addressing the problem in this letter to the believers there.
He began his letter noting his thanks to God for the grace and spiritual gifts given them. And after reminding them of God's faithfulness in calling them into fellowship, Paul appeals to them "in the name of our Lord Jesus Christ." He calls them to "agree with one another," and he admonishes them, demanding an essential unity of "mind and thought." There was to be no room for divisions. Instead, love, unity, and harmony are to be the signs of Christ's presence.
When believers forget what unites them, they focus on externals such as who baptized whom, or who follows which leader, or who comes from what nation, or who has what skin color, or who has the most money. Our harmony is not to be based on our common preferences or interests. Ego-driven divisions have no place among us. Our allegiance to Christ must be expressed in our love for one another. It's essential!