Our third reason for not doing prayers for healing in evangelism is that "It's not guaranteed":
The Bible doesn’t say anywhere that it is God’s will to always heal a person, in fact Scripture says that God is the one who causes or permits diseases or sicknesses to happen (Exodus 4:11; Luke 1:19–20).
Therefore we cannot ever guarantee that a person will be healed if we pray for him, because God can say no to the prayer - just like God said no each time to Paul’s prayer about removing the thorn of the flesh (2 Cor. 12:8).
Or how Paul had to leave Trophimus behind at Miletus because he was ill (2 Timothy 4:20) - despite undoubtedly praying for his healing.
We then as believers should never advertise, as some do, “free healings”. We are not a magician nor are we God. We cannot even make one of our hairs white or black (Matt. 5:36).
But there is something that we can guarantee to unbelievers - that whoever believes in Christ will not perish but have eternal life. For that is a promise of God. So let’s be focused on proclaiming that.