Christ's challenge to us
When we hear Christ admonish his disciples for lack of faith, instead of feeling like those apostles haven't yet reached our own personal stature, we need to consider what Christ considered someone who had faith like a mustard seed to be capable of.
Luke 17: 6 And the Lord said, If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you.
If we were there with them, would Christ have changed his correction from "why are ye of little faith" to "Peter, why don't you have the faith of brother so-and-so standing here next to me?"
When we read Christ tell Peter to strengthen his brethren when Peter is converted, we need to ask whether we are converted enough to cast out devils and heal in the name of Jesus in the same miraculous way Christ's apostles were known for doing. We need to ask whether we have faith to walk on water, or to say to a sycamine tree "Be thou plucked up by the root, and be thou planted in the sea;" and it would obey us. If not, we should read these corrections to the ancient apostles as applying equally well to us, as we do not yet have faith like a grain of mustard seed, and we are not yet truly converted.