This is one of the most hypocritical teachings that men have brought forth in the name of the Bible. Do not lose your faith, or you will lose your healing. Keep believing, or that devil will come back on you.
Faith healers do plenty of good when they boost people's faith enough for healing, but they trample over much more people who have medical conditions that God has allowed them to have. Not every person in the world is going to be healed by God. In fact, a limited few may be healed in order to boost the faith of the others, but the numbers of the sick and afflicted outweigh the numbers of those healed by a large margin.
I have family members who condemn their own hearts because they cannot understand why God does not heal them. Their faith must be lacking, or they must be sinners.
WMB and G. Lindsay were by far the largest instigators of this belief in the world today. Take this excerpt from one of WMB's pamphlets sold to people who came through the prayer lines:
The Important Matter Of Keeping One's Healing
This last consideration brings us to the thought of whether it is possible to lose one's healing. This matter is really of tremendous importance, for we have already seen that Jesus plainly taught that it is possible for one to lose his healing. Almost every Christian will testify that after they were saved, glorious as their experience might have been, sooner or later the enemy was back to tempt and to even make them believe that they had not been saved. Likewise, as we have seen in the case of demon possession, the demon is back sooner or later to recover, if possible, what he has lost. To be forewarned is to be forearmed, if the individual will obey the Scriptures by resisting the enemy, the devil will leave, but if he gives in to him, Satan will rob him of his deliverence. James 4:7 declares, "Resist the devil and he will flee from you." If when the enemy brings symptoms to us, tempting us to believe that the old affliction is to return upon us, we resist him and rebuke the thing, we shall find that our deliverance is complete.
We must let Christ and His Kingdom reign in our lives. The man in whom the demon was cast out, continued in his old way. He did not serve God or live for Him. Consequently, his disobedience was an invitation for the enemy to return. There are those who only attend church during a revival meeting. They are unfaithful in their service to God, and continue to indulge in a worldly life, partaking in questionable things on the devil's territory. Soon they may be back in the toils of the enemy again. Again, let us remember the words of Our Lord when He said, "Behold thou art made whole; sin no more, lest a worse thing come unto thee."
There are, however, numbers of good Christians, conscientious people who evidently receive a gracious touch from the Lord, but for some reason or other do not seem to get a complete healing. To those who have had certain types of disease, special instructions need to be given. In the case of a cancer patient, the cancer is usually killed after prayer. Nevertheless, in some cases the cancer has yet to be eliminated from the body. Sometimes severe pain attends this elimination. If the person fails to understand this, he may fail to recognize his healing altogether. However, the person who stands firm in faith may expect complete deliverance.
In one of the Branham meetings, a lady had a serious cancer that was prayed for. Early the following morning she was visited with severe pains, and became fearful. At 5 a.m. in the morning she sought to obtain help by telephone, but was unable to secure proper connections. Shortly after, however, she passed the cancer from her body and the next day was up testifying to her healing.
What is true concerning the action of a dead cancer is similarly true with other growth afflictions. People who have had growths in their ears which caused deafness, often after hearing for a little time, suffer temporary deafness again while the growth disintegrates. The same things is true of cataract blindness. The cataracts which have been killed, during the period of drying up, may produce temporary blindness again. To all these we say, trust God and stand upon His promise for complete deliverance. Let not that man think that he shall receive anything of the Lord." (James 1:6-7.) But let us remember the words of Jesus in Mark 10:24: "What things soever ye desire, when ye pray, believe that ye receive them and ye shall have them."
Gordon Lindsay - Divine Healing in the Branham Campaign
If you have a medical condition, and God has not healed you, then you are the perfect candidate for modern medicine. Jesus did not tell the people to go on their way while warning them the disease might come back on them -- they were made completely whole. If they had the faith to be healed in the first place, Jesus would have chosen others with less faith to heal.
Everything on this earth is made for the glory of God. Medicine has been given to us for the glory of God. If we reject the things God has given us, are we rejecting the glory of God?