In Peter we have a striking mirror of our ordinary condition. Many have an easy and agreeable life before Christ calls them; but as soon as they have made profession of his name, and have been received as his disciples, or, at least, some time afterwards, they are led to distressing struggles, to a troublesome life, to great dangers, and sometimes to death itself. This condition, though hard, must be patiently endured. Yet the Lord moderates the cross by which he is pleased to try his servants, so that he spares them a little while, until their strength has come to maturity; for he knows well their weakness, and beyond the measure of it he does not press them. Thus he forbore with Peter, so long as he saw him to be as yet tender and weak. Let us therefore learn to devote ourselves to him to the latest breath, provided that he supply us with strength.
In this respect, we behold in many persons base ingratitude; for the more gently the Lord deals with us, the more thoroughly do we habituate ourselves to softness and effeminacy. Thus we scarcely find one person in a hundred who does not murmur if, after having experienced long forbearance, he be treated with some measure of severity. But we ought rather to consider the goodness of God in sparing us for a time. Thus Christ says that, so long as he dwelt on earth, he conversed cheerfully with his disciples, as if he had been present at a marriage, but that fasting and tears afterwards awaited them, (Matthew 9:15.).
~John Calvin, comment. on John 21:18