Each one of us must understand how to receive healing and keep it. When we understand God's word on healing and act upon it, it brings forth a manifestation of healing in our physical bodies. Our goal is to receive the manifestation of healing and give our heavenly Father the glory for what Jesus has done for us.
Sickness is the result of an oppression of the devil
The devil inflicts us with sickness and doesn't like it when God's Word brings healing and destroys his work. The devil will always try to reestablish his infliction by stealing the truth of the Word from us. We see this in Matthew 13:19 where it says, "When anyone hears the Word of the kingdom, and does not understand it... the evil one comes and snatches away what has been sown in his heart. This is the one on whom seed was sown beside the road."
The enemy wants to steal the Word before it can become firmly rooted in us to produce the healing. The devil knows that the Word of God is more powerful than his work ... sickness, so he must try and cut off the Word in us before it gets rooted. Only seed rooted can produce a harvest.
Let's look at Matthew 13:20-21
When we receive the Word on healing and are prayed for and a healing takes place, we receive it with joy, but it must become firmly rooted in us because the devil will always try to inflict us with the very same symptoms to change our minds. Verse 21 says because of the Word, this affliction will be brought against us once again to cause us to fall away. The devil will once again bring the symptoms of the sickness to try and convince us that we are not healed. This is a lying vanity. The root meaning of a lying vanity is something that comes against you to change your mind. If the devil can cause us to look at symptoms instead of God's promise, he can convince us to believe his lie. When we focus on the symptoms, pain or whatever it is, doubt and unbelief come to our mind and this gives entrance for fear to reestablish the sickness. We must keep our heart and mind fixed upon God and His promise.
We need to hold fast to that which is good. We must allow the Word to take root so that it can complete the manifestation of healing.
Jonah is an example of this. He was swallowed by the whale and was in the whale's belly for three days and nights, yet he did not give up on God and His promise.
He said in Jonah 2:8-9
Jonah decided not to believe a lying vanity, but to trust God and God delivered him out of his circumstance. Had Jonah believed the lie, he would have lost his faith and not received deliverance. How can we expect God to deliver us if we go by what the devil does rather than trust God?
So when the symptoms return, we must set our faith in God and his Word and tell the enemy, no matter what, we are not going to believe his lies.