At the time of the Reformation, some persons who claimed to be Reformed pretended to be Roman Catholic, attending mass and giving their Romanist neighbors no reason to think that they were Reformed. Calvin addressed this behavior, calling on these people, if they believed in Christ, to make a public confession of their faith, rather than dissembling for the purpose of keeping their status in society. He called them to separation from a false church.
John Calvin (1509-1564) was a theological giant of the Protestant Reformation. A contemporary of Martin Luther, he had as much influence over this period of history as his German counterpart. In 1536 he published his famous Institutes of the Christian Religion, which was a systematic presentation of the Protestant position. His writings are still cherished and relevant today.