I was surprised to read a short letter from John Calvin written to Laelius Socinus. It was Laelius Socinus's rationalism that caused him to doubt many orthodox beliefs, often in the name of reason, and as a radical reformer of the 16th century he pioneered the ideas that his disciples formalized as the famous antitrinitarian heresy Socinianism. Laelius Socinius's nephew Fausto Sozzini formalized these ideas established this famous antitrinitarian 17th century sect. However, it was Laelius Socinus's proto-Socianian ideas addressed in this letter from John Calvin that led to this antitrinitarian heresy that is alive and well to this day. Laelius Socinus had written to John Calvin multiple times and as well as to Ulrich Zwingli's disciple Hendrich Bullinger. Laelius Socinus had received multiple responses from the patriarchs of the Reformation, and had even uses those response to influence others (as with the example of Melanchthon below.) I came across this short letter in a compendium I read recently called The Letters of John Calvin (published by Banner of Truth.)

This founder's correspondence with John Calvin is the most interesting part of the letter (especially the warnings in it). For a great response to Socinianism, see Francis Turretin's Institutes of Elenctic Theology for a detailed rebuttal Faustus Socinus. Faustus Socinus was to Turretin, as Servetus was to John Calvin.

CCLXXXIX. – To Lelio Socin.

Refusal to reply to the curious questions proposed to him by Socin.
[1551 A.D.]

PREFACE

Lelio Socin, founder of the celebrated sect which bears his name, was born in Sienna of a distinguished family: his father, Mariano Socin, a professor in the University of Bologna, was one of the most learned jurisconsults of his age. Of a bold and active mind, which found pleasure in the most subtle speculations, and which would not stop short of the interpretation of mysteries, Lelio left his native country in 1548, and joined the Reformers of Switzerland and Germany, whose friendship he won by the politeness of his manners, the purity of his life, and his zeal for learning. He resided by turns at Zurich and Wittenberg, and was not slow, by correspondence or conversation, to express his doubts on the common doctrines, which he skillfully advanced rather in the form of questions than as opinions, which he was prepared to maintain and teach. He was beloved by [Hendrich] Bullinger, who did not suspect the heterodoxy of his beliefs, and who wrote to Calvin himself, after having repeatedly broken off correspondence with Socin, could no forbear renewing it, and giving a friendly reply to the doubts which he had expressed on the resurrection, baptism, the trinity, &c. (Calvin. Opera, tom. ix. pp. 51, 57, 197.) The letter, which is published here for the first time, throws valuable light on the relation of the Reformer to the founder of a sect to which even Socin himself was yet a stranger, and whose doubts were afterwards to be setup as dogmas by his disciples. Lelio Socin died in 1562, before he had completed his thirty-seventh year. –M’Crie, Hist. of Ref. in Italy, passim.

LETTER

You are deceived in so far as you entertain the impression that Melanchthon does not agree with us on the doctrine of predestination. I only said briefly that I had a letter written by his own hand, in which he confessed that his opinion argreed with mine. But I can believe all you say, as it is nothing new for him to elude in this matter, the better to rid himself of troublesome inquiries. Certainly no one can be more averse to paradox than I am, and in subtleties I find no delight at all. Yet nothing shall ever hinder me from openly avowing what I have learned from the word of God; for nothing but what is useful is taught in the school of this master. It is my only guide, and to acquiesce in its plain doctrines shall be my constant rule of wisdom. Would that you also, my dear Lelio, would learn to regulate your powers with the same moderation! You have no reason to expect a reply from me so long as you bring forward those monstrous questions. If you are gratified by floating among those aerial speculations, permit me, I beseech you, an humble discipline of Christ, to meditate on those things which tend towards the building up of my faith. And indeed I shall hereafter follow out my wishes in silence, that you may not be troubled by me. And in truth, I am very greatly grieved that the fine talents which God has endowed you, should be occupied not only with what is vain and fruitless, but that they should also be injured by pernicious figments. What I warned you of long ago, I must again seriously repeat, that unless you correct in time this itching after investigation, it is to be feared you will bring upon yourself severe suffering. I should be cruel towards you did I treat with a show of indulgence what I believe to be a very dangerous error. I should prefer, accordingly, offending you a little at present by my severity, rather than you allow to indulge unchecked in the fascination allurements of curiosity. The time will come, I hope, when you will rejoice in having been so violently admonished. Adieu, brother very highly esteemed by me; and if this rebuke is harsher than it ought to be, ascribe it to my love to you.

[Lat copy. – Library of Geneva. Vol. 107, a.]

pg 330-331, The Letters of John Calvin 

The names may be confusing due to the variant spellings in different language. For instance, Laelius Socinius is addressed as "Lelio Socin" in this letter, and he is name is elsewhere latinized as Laelius Socinius. The latinized form of Fausto Sozzini is Faustus Socinus, and Francis Turretin is also known as François Turretini.