Jesus died to buy healing for mankind. Should I rather say, healing potential, because there is faith involved in healing. "Surely He has borne our griefs and carried our sorrows; (sicknesses).." Isaiah 53:4a I inserted the parenthesis because if you look up the Hebrew word for "sorrows," the original word is sicknesses. The Righteous Son of God carried our sins and our sicknesses. This means we have a right to walk free of both. The Cross brought forth a legal position for mankind, but legalities must be enforced. We enforce our rights under the Cross of Jesus. If we do not stand up and enforce what has been given to us as a free gift, our enemy and God's enemy, the devil will come in and run over us and steal our blessing and our rights in the Cross of Jesus. Jesus said in John 10 that, "the thief came to steal, kill and destroy." Then, He said, "I am come that you might have life, and life more abundantly."
So when sickness, which is of Satan, and not of God, comes, we must use the prayer of faith. That is not a shot gun prayer thrown at Heaven. It is a laser beam like prayer prayed by seasoned people who are tested in faith and cannot be tempted to doubt. God said call for the elders. Those are supposed to be seasoned people in God who know how to pray for the sick. "Is anyone among you sick? Let him call for the elders of the church, and let them pray over him, anointing him with oil in the name of the Lord. And the prayer of faith will save the sick, and the Lord will raise him up." James 5:14-15a
Many modern elders cannot pray the prayer of faith, because they never learned how. You must find one who does. But, even better, is to learn the prayer of faith from someone and then use it yourself, for this is God's highest desire for His children.
NOW, KEEPING THE HEALING:
Listen to this scripture: "Afterward Jesus found him in the temple, and said to him, “See, you have been made well. Sin no more, lest a worse thing come upon you.” John 5:14 Jesus was healing under the Old Covenant. He lived on earth during the Old Covenant. The New Covenant began at the Cross when invisible hands tore the veil in the temple as Jesus died. Jesus at that moment re-united man with God for all time. On this side of the Cross, in this New Covenant, Jesus would probably not say, "Sin no more, lest a worse thing come upon you."
But a principle still applies. James says, "that which is not of faith is sin." God views any unbelief we speak or act on, or any fear, as sin. God makes us righteous by faith. Righteousness is reckoned unto us by our faith. (Romans 4) Without faith, we cannot please God. (Hebrews) If I may give a modern paraphrase to Jesus' comment to that man, I believe you will see the truth to stay well and keep a healing.
"So you have been healed. Here is a warning. When you get the word sown into your heart, Satan comes immediately to steal that word from your heart. (Mark 4) I say to you, stay believing, and if any symptoms return, go do what you did before, use the prayer of faith again, stand fast, do not doubt or speak unbelief or fear, only believe, and do not look at circumstances, or what you see with the sight of your eyes, but trust God, and forcefully enforce your rights from Calvary. Do it again and again until evil gives up, because you are way too persistent for him. If you do this, the thing will not come back to stay, and no worse thing will come upon you, for you fixed the real thing, which was not the sickness, but your own unbelief."