The more I read of Calvin, the more I see that it is really a different animal from Lutheranism--even of some concepts overlap. I don't mean to simply condemn Calvin and Calvinists, of course. It is just that the difference is plain every time I read his writings. For one, he is far more likely to say that something cannot be true because it offends reason than Luther was or than Lutherans in general are. Having said that, he is not as rigorously rational as I have believed. I don't get the sense that his rigorous logic gives way to mysticism or symply not resolving paradoxes, I mean that he is rigorously logical when it suits his philosophical axioms but he is quite willing to abandon strict logic when his axioms call for it. An example was his claim that the stone at the tomb rolled away when Jesus walked out, and then rolled back after he left, only to roll away again to show the empty tomb. (Calvin Institutes IV 17:29)
In any case, to my knowledge Lutherans have always taught that the Son, i.e. the eternal second person of the Trinity, is eternally begotten by the father throughout all time, not as a sort of one-time event before time. In this Lutherans more or less what historic Christianity teaches. However, Calvin has his doubts:
I hope the pious reader will admit that I have now disposed of all the calumnies by which Satan has hitherto attempted to pervert or obscure the pure doctrine of faith. The whole substance of the doctrine has, I trust, been faithfully expounded, if my readers will set bounds to their curiosity, and not long more eagerly than they ought for perplexing disputation. I did not undertake to satisfy those who delight in speculate views, but I have not designedly omitted anything which I thought adverse to me. At the same time, studying the edification of the Church, I have thought it better not to touch on various topics, which could have yielded little profit, while they must have needlessly burdened and fatigued the reader. For instance, what avails it to discuss, as Lombard does at length, (lib. 1 dist. 9,) Whether or not the Father always generates? This idea of continual generation becomes an absurd fiction from the moment it is seen, that from eternity there were three persons in one God.(Calvin, Institutes I 13.29) [emph. added]
The eternal generation is not questioned so much because it is unbiblical as because it is absurd.
In my opinion, Calvin's admittedly uneven application of reason leads him down some theological dead ends, such as making Christ a person out of two natures, double predestination and his denial of the Real Presence, or the effacasy of Baptism. He is a little fuzzy on assurance--i.e. can or will a believer know he is elect, but even when he does speak of assurance he has a tendency to emphasize the quality of one's faith.
On all these points he differs from Lutheranism.