Calvin labored long and hard to promote church unity. His goal was to bring together all true believers into one communion. He once wrote a letter to Archbishop of Canterbury Thomas Cranmer to propose an assembly of the most eminent men of learning from all the various churches that had embraced the pure doctrine of the gospel, in order that they might make careful study of the Word of God and then draw up a true and distinct confession to which all might subscribe.
Unity and unanimity, he said, are necessary if the church wants to persist in this world. Yet spiritual unity does not require uniformity. He understood that the unity Christ wants His people to pursue is first of all a unity in the truth (John 17). Such unity, Calvin believed, was attainable even if some doctrinal differences continued to exist. As he explained:
For not all articles of true doctrine are of the same sort. Some are so necessary to know that they should be certain and unquestioned by all men as the proper principles of religion. Such are: God is one; Christ is God and the Son of God; our salvation rests in God’s mercy, and the like. Among the churches there are other articles of doctrine disputed which still do not break the unity of the faith.… First and foremost, we should agree on all points. But since all men are somewhat beclouded with ignorance, either we must leave no church remaining, or we must condone delusion in those matters which can go unknown without harm to the sum of religion and without loss of salvation.22
22 Institutes 4.1.12.
 Pronk, Cornelis. “Calvin’s Doctrine of the Church.” Calvin for Today. Ed. Joel R. Beeke. Grand Rapids, MI: Reformation Heritage Books, 2009. 148–149.