A Voice from the Past - Calvin

For all the heads of true doctrine are not in the same position. Some are so necessary to be known, that all must hold to them to be fixed and undoubted as the proper essentials of religion; for instance, that God is one, that Christ is God, and the Son of God, that our salvation depends on the mercy of God, and the like. Others, agai,n which are the subject of controversy among the churches, do not destroy the unity of the faith; for why should should it be regarded as a ground of dissension between churches, if one, without any spirit of contention or perverseness in dogmatizing, hold that the soul on quitting the body flies to heaven, and another, without venturing to to speak positively as to the abode, holds it for certain that it lives with the LLord?

John Calvin, 1509-1564, Institutes of the Christian Religion, Book IV, Chapter 1, 12, (translated by Henry Beveridge, Wm. B. Eerdmans Publishing, 1975, Vol. II, p. 291).

How do unnecessary things become a cause of division in the Christian church? How could this be avoided?