John Calvin:

“I hear that Luther has at length broken forth in fierce invective, not so much against you as against as against the whole of us. On the present occasion, I dare scarce venture to ask you to keep silence, because it is neither just that innocent persons should thus be harassed, nor that they should be denied the opportunity of clearing themselves; neither, on the other hand, is it easy to determine whether it would be prudent for them to do so.

But of this I do earnestly desire to put you in mind, in the first place, that you would consider how eminent a man Luther is, and the excellent endowments wherewith he is gifted, with what strength of mind and resolute constancy, with how great skill, with efficiency and power of doctrinal statement, he hath
hitherto devoted his whole energy to overthrow the reign of Antichrist, and at the same time to diffuse far and near the doctrine of salvation. Often I have been wont to declare, that even although he were to call me a devil, I should still not the less hold him in such honor that I must acknowledge him to be an illustrious servant of God.

But as he is endowed with great virtues, so he labors under great failings…It is our duty to reprehend what is evil in him, in such a manner as to yield very much to his excellent qualities. Consider, I beseech you that you have to deal with a chief servant of Christ to whom we are all indebted. And then, that by contending that you will effect nothing but a pleasure to the impious, who will triumph, not so much over us, but as over the gospel. “