I recall an old slogan:
“In essentials, unity.
In nonessentials, diversity.
In all things, love!”

The early bishop and martyr Polycarp, whose feast is celebrated on Feb 23, was involved in a dispute that is a model of Christian brotherhood in the midst of disagreement. The dispute is known by the obscure name “Quartodecimanism” from a Latin term meaning “fourteenth”.

The controversy arose because Christians in Jerusalem and Asia Minor, following guidance from the Apostle John, chose to celebrate Passover on the 14th day of the “first month”. They felt that the crucifixion of Jesus should carry the emphasis, and that this day should be the principle feast for Christians. On the other hand, churches in and around Rome had changed the principle celebration to the following Sunday (as is the commonplace today for most Christians). The dispute became quite heated at times, leading almost to excommunications.

We have a record of how Polycarp and his opponent treated each other on this issue:

And when the blessed Polycarp was at Rome in the time of Anicetus, and they disagreed a little about certain other things, they immediately made peace with one another, not caring to quarrel over this matter. For neither could Anicetus persuade Polycarp not to observe what he had always observed with John the disciple of our Lord, and the other apostles with whom he had associated; neither could Polycarp persuade Anicetus to observe it as he said that he ought to follow the customs of the presbyters that had preceded him. But though matters were in this shape, they communed together, and Anicetus conceded the administration of the eucharist in the church to Polycarp, manifestly as a mark of respect. And they parted from each other in peace, both those who observed, and those who did not, maintaining the peace of the whole church.

(Eusebius, quoting a letter by Irenaus, available at earlychurchtexts.com)

Eventually the debate was settled, and the Roman practice prevailed, though a few holdouts persisted into the fourth century. Today Easter Sunday is the biggest feast on the Calendar for Christians throughout the world.

Other sources:
Campbell, T. (1907). Pope St. Anicetus. In The Catholic Encyclopedia. New York: Robert Appleton Company. Retrieved February 25, 2018 from New Advent: http://www.newadvent.org/cathen/01514a.htm