Adam Clarke on Luke 17:14

Show yourselves unto the priests - According to the direction, Lev 13:2, etc.; Lev 14:2, etc. Our Lord intended that their cure should be received by faith: they depended on his goodness and power; and though they had no promise, yet they went at his command to do that which those only were required by the law to do who were already healed.

And - as they went - In this spirit of implicit faith; they were cleansed. God highly honors this kind of faith, and makes it the instrument in his hand of working many miracles. He who will not believe till he receives what he calls a reason for it, is never likely to get his soul saved. The highest, the most sovereign reason, that can be given for believing, is that God has commanded it.