In Matt. 8, the centurion believed so much in Jesus that he said, "Only speak the word, and my servant would be made whole". Jesus replied that it would be done onto him as he had believed. Jesus has already made the general declaration that if one has faith like a mustard seed, when he declares onto a mountain to move, it shall move.
Hence Christians should tap into this opportunity and have belief (faith). Since Jesus has already made the declaration, things would be done according to the faith of the one believing. Jesus said that for the one who has faith, nothing shall be impossible for him or her. This means that we can even obtain things which God may not will for us to have at that moment. In Isaiah 38, God told King Hezekiah about his impending death. When he prayed with faith to God, fifteen more years were added onto him.
Unbelief should never ever be part of the Christian. Jesus willed to heal the centurion's servant at His (Jesus') own convenience. That was Jesus' will at that time. But due to the centurion's faith, He (Jesus) was compelled to do it immediately. It should be noted however that "faith cometh by hearing, and hearing by the word of God" Rom. 10:17 (KJV). Christians should have their own faith innovations (i.e. in whatever situation they want). But when the faith deed comes to pass, they should know that the power is from God and not from man. Amen!!!!