There are a disturbing number of Christians out there who doubt each other's faith based entirely on their circumstances. The Bible repeatedly promises trials and tribulations for believers, and assures us that these things work out for the good (Romans 8:28), that they lead to perseverance, character and hope (Romans 5:3-5), that they cause us to rely on the God who raises the dead (2 Corinthians 1:9), and that in all of this, God's grace is sufficient for us, and His strength is made perfect in our weaknesses (2 Corinthians 12:9). In other words, there is nothing deficient in the faith of someone who is sick, financially troubled, persecuted, or even dying.
And yet, many Christians will claim that if a believer is sick, that believer simply hasn't got enough faith. If you were a real Christian, you wouldn't pray "if it is your will", you would actually be powerful enough to rebuke that sickness.
The leper in Luke 5 didn't seem to agree with them. Faced with the Lord Himself, he said "Lord, if you are willing, you can make me clean". If. If you are willing. And Jesus didn't rebuke him for this. He seems to do so to the man in Mark 9:23 (admittedly, this seems to vary between translations), but rather than say to the leper "you don't have enough faith, therefore I cannot heal you", or try to coax out a stronger confession of faith, He simply told him "I am willing" and healed him.
For the Christian, it should not be even remotely logical to doubt that God can do something, much less to believe that He would be hindered by something so fragile as our faith. Let's be honest here, if God was so limited by humanity, we'd have taken over the throne long ago. But the question is how does He allocate this power? Answer: How He wills. For the Christian, a "no" from God should always be considered a valid answer.
As I always say, prayer is a servant-master relationship, but we need to understand who is the servant and who is the master. You may have a thorn in your flesh, and you may pray three times that it might leave, if God is not willing, no amount of faith will make it go away. Paul learned this the hard way in 2 Corinthians 12:7-10, where he wrote "And lest I should be exalted above measure by the abundance of the revelations, a thorn in the flesh was given to me, a messenger of Satan to buffet me, lest I be exalted above measure. Concerning this thing I pleaded with the Lord three times that it might depart from me. And He said to me, “My grace is sufficient for you, for My strength is made perfect in weakness.” Therefore most gladly I will rather boast in my infirmities, that the power of Christ may rest upon me. Therefore I take pleasure in infirmities, in reproaches, in needs, in persecutions, in distresses, for Christ’s sake. For when I am weak, then I am strong."
My fellow Christians, I ask in the name of our Lord that this cease to become an issue of such contention. To my sick brethren, whether you have so much as a cut on your finger, a scar on your stomach, a missing limb, or a life threatening illness, let no one tell you "you just don't have enough faith". You're not deficient in faith! God has elected to allow you to be afflicted, and in this affliction, you must rely on Him, and be committed to glorifying Him.
To my faith-healing brethren, stop disputing the faith of the sick. By guaranteeing a healing that might never come, you are shaking the faith of these people, causing them to stumble, making a mockery of Christianity to the outside world, and may even be committing blasphemy by assuming Christians have the power to control God rather than submit to Him in sickness and in health.