Paul writes the Corinthians about what makes for unity.
Quarrelling and factions are symptoms he identifies,
Symptoms that show something is wrong.
Unity is found in the central message of Christianity:
The Word of the Cross of Christ.
His life, teaching, death and resurrection; His promised return.
"The word of the cross is folly to those who are perishing, but to us who are being saved, it is the power of God."
Foolishness of the Gospel pt 1
Foolishness of the Gospel pt 2