"If you can?" said Jesus. "Everything is possible for one who believes."
What is impossible to God? With God, all things are possible. Things are not always as they seem in the hands of Jesus. God is able to step into any situation at any time of His choosing and change the course of our lives. Our Lord Jesus said these words to remind us of that fact, and to also fuel our faith in the impossible, for certainly His own resurrection would be impossible for those without faith.
Therefore, He drives the point home and says for those of faith, everything is possible. No doors are closed, not even death. God can do all things. He is Almighty, and He is sovereign over His universe. Throughout Scripture God reminds us of this, not to fear, not to doubt, but to always believe. This is a childlike heart of faith, and this is the heart to which the impossible is quite possible.