"'Have faith in God,' Jesus answered. 'Truly I tell you, if anyone says to this mountain, "Go, throw yourself into the sea," and does not doubt in their heart but believes that what they say will happen, it will be done for them. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours. And when you stand praying, if you hold anything against anyone, forgive them, so that your Father in heaven may forgive you your sins.'" NIV translation
Jesus met Peter's wonder with a simple lesson on prayer and authority. He was amazed at the tree so imagine what he must have thought when Jesus said that he could make a mountain move. The lesson gets a little harder when Jesus spoke of forgiveness. We remember that during their trip to Jerusalem there was some bad feelings between the disciples over James and John's request. Was Jesus saying that they need to let that go? For us, even as Christians, true forgiveness is only possible through the power of the Holy Spirit. That is why Jesus said that it was necessary for God to forgive us of our sins. The Holy Spirit is a sort of identification card that says we are a child of God.