But that which gave the Lutherans the most trouble in this century was the efforts of a succession of persons to bring about a state of harmony between them and the Calvinists.
James I of England tried it, and failed.
In 1631, in a synod of the Calvinists at Charenton, an act was passed, which granted that the Lutheran religion "was conformable to a spirit of true piety, and free from pernicious and fundamental errors," but the overture was not accepted.
In the same year, a conference was held at Leipsic, between several of the most eminent doctors of both communions, in Saxony and Brandenburg. And although the Calvinists showed all possible fairness, and made concessions that the Lutherans themselves could scarcely expect, yet all their efforts were looked upon and regarded with suspicion, as being only schemes to ensnare them; and the conference broke up with nothing done.
In 1645 Udislaus IV, king of Poland, called a conference at Thorn, but it only increased the party zeal.
In 1661 William VI, landgrave of Hesse, called a conference at Cassel, in which the doctors there assembled came to an agreement, embraced one another, and declared that there was nothing between them of sufficient importance to prevent union and concord. This was no sooner learned by the Lutheran brethren, than they turned all their fury against their delegates, and loaded them with reproaches of apostasy, Calvinism, etc.
Besides these public efforts, there were others of a private character. John Duraeus, a Calvinist, a native of Scotland,
"...during a period of forty-three years, suffered vexations, and underwent labors which required the firmest resolution, and the most inexhaustible patience; wrote, exhorted, admonished, entreated, and disputed; in a word, tried every method that human wisdom could suggest, to put an end to the dissensions and animosities that reigned among the Protestant churches..He traveled through all the countries in Europe where the Protestant religion had gained a footing; he formed connections with the doctors of both parties; he addressed himself to kings, princes, magistrates, and ministers..But his views were disappointed. .Some, suspecting that his fervent and extraordinary zeal arose from mysterious and sinister motives, and apprehending that he had secretly formed a design of drawing the Lutherans into a snare, even attacked him in their writings with animosity and bitterness, and loaded him with the sharpest invectives and reproaches: so that this well-meaning man, neglected at length by his own communion..spent the remainder of his days in repose and obscurity at Cassel." (Church History, 17th cent., sec. 2, part 2, chap. 1, paragraph 6)
That which he proposed as the foundation upon which they might unite, was, The Apostles' Creed, The Ten Commandments, and The Lord's Prayer.
Another of the most zealous of the peacemakers was John Matthias a Swedish bishop, who with George Calixtus, attempted to carry on the work of Duraeus. But the opposition was so bitter that Matthias was obliged to resign his bishopric; Calixtus was accused, and to his
"charge many other things were laid, besides the crime of endeavoring to unite the disciples of the same Master in the amiable bonds of charity, concord, and mutual forbearance." (Id. Par. 7)
This "crime" was called syncretism.