"And the Lord said, Simon, Simon, behold, Satan hath desired to have you, that he may sift you as wheat:
But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren."
Simon Peter here had spent a lot of time with Christ. He had walked out of a boat, on water, for a few steps, trying to emulate Jesus and become more like him. He had listened to him and obeyed him for years. And yet, here Christ infers that he hadn't yet been converted. I think this is an important lesson.
Conversion is a huge step in our lives, and for some of us it happens before baptism. That's probably why people who join the church as adults are often called "converts," because they experience a conversion before they are willing to join the church. But conversion for many of us doesn't happen that quickly, or that obviously. Often, we're like Simon Peter here, following and figuring it out, but without the "mighty change" (Mosiah 5:2, Alma 5:14) of conversion that goes deeper than simply believing and following can go. Sometimes we sit in church and wonder why we haven't had the experiences other people have had, or why we don't feel answers to prayers the same way. Peter became the head of the church after Christ's crucifixion. It's not a bad thing to be where Peter was, but as it shows in this scripture, it made him vulnerable to Satan. We are similarly vulnerable until we are converted, and even after if we don't maintain those feelings (Alma 5:26).
So, how do we get there... past following and believing into having our hearts changed? I don't think it is exactly the same for everyone, but it's similar to what happened to Simon Peter. Instead of just following and believing, we have to have faith to act on our beliefs, to stand up for Christ everywhere we are... to not be ashamed of being associated with him. We have to pray, and work, and sincerely want to be changed from who we are into who God would have us be. That's a tough thing sometimes... to be willing to give up our old selves to become better selves. But it's also a necessity for true conversion. God changes our hearts, but he won't do it against our will. Today, let's be willing to have our hearts changed and to become more than we are. Let's be willing to invest our whole selves and become converted.