"[Peter] was kind-hearted, quick, ardent, hopeful, impulsive, changeable, and apt to run from one extreme to the other.....He protested, in presumptive modesty, when Christ would wash his feet; and then, suddenly changing his mind, he wished not his feet only, but his hands and head to be washed. He cut off the ear of Malchus in carnal zeal for his Master; and in a few minutes afterwards he forsook him and fled. He solemnly promised to be faithful to Christ, though all should forsake him; and yet in the same night he betrayed him thrice." -Phillip Schaff

Ah, loveable Peter. Changeable as the weather of Kansas, either all in or all out, a man of extreme loves and hatreds.  He changed his mind at the drop of a hat, and he meant it when he did; but no matter how much he meant it in the moment, it did not guarantee steadfastness of purpose.

And yet, Christ chose this mutable disciple, the most volatile, impulsive one of all, and named him "Rock". Hard and fast, rocks don't change, at least not quickly, and are not subject to outside influences. They are steady, they make a good foundation because they don't shift or change. And then, not only did he change his name to Petrus, he called him to be the rock of the church.

It's as if Robin Hood had not only named the giant John "Little John" but THEN asked him to crawl through a child-sized window.

God does this with Moses in the Old Testament, too. He calls him to speak to Pharoah, calls Moses,he guy who can't talk without stumbling over his words, to speak to the king, overlooking Aaron, who was skilled in speech. 

"Then Moses said to the LORD, "Please, Lord, I have never been eloquent, neither recently nor in time past, nor since You have spoken to Your servant; for I am slow of speech and slow of tongue." The Lord said to him, "Who has made man's mouth? Or who makes him mute or deaf, or seeing or blind? Is it not I, the LORD? Now then go, and I, even I, will be with your mouth, and teach you what you are to say." But he said, "Please, Lord, now send the message by whomever You will." Then the anger of the Lord burned against Moses, and He said, "Is there not your brother Aaron the Levite? I know that he speaks fluently. And moreover, behold, he is coming out to meet you; when he sees you, he will be glad in his heart. You are to speak to him and put the words in his mouth." God let Moses have his wish, but He was angered that Moses was unable to look past himself and put his trust in God.

God calls us to do things that we cannot do. 

He calls us to be patient, He calls us to be kind, he calls us to be chaste and diligent. He calls us to rejoice with others. He calls us to weep with others. He calls us to speak the truth in love. He calls us to think of others more highly than ourselves. He calls a woman in her tenth decade to motherhood. He calls a virgin to bear the Savior of the world.

 He gives more grace when the burden grows greater. His strength is perfected in our weakness. When we can't, He can, and He will. And when He does, all glory be to Him. So when He calls, hand yourself over to Him, in all your failings and weaknesses and insecurities, and and His power will be made known through you.