Consider Christ's intercessory prayers. See the Preservation of the Saitrhs in this?
31 And the Lord said, Simon, Simon, behold, Satan hath desired to have you, that he may sift you as wheat:
32 But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren. Luke 22:31-32.
Notice Jesus didn't say, "if you return" but, "when you return."
Also,
11 And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are.
15 I pray not that thou shouldest take them out of the world, but that thou shouldest keep them from the evil.
24 Father, I will that they also, whom thou hast given me, be with me where I am; that they may behold my glory, which thou hast given me: for thou lovedst me before the foundation of the world. John 17.
How could the prayers of Christ ever fail?
25 Wherefore he is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them. Hebrews 7.