John Lightfoot on Luke 22:32

[That thy faith fail not.] There seems an emphasis in the word faith. As to the other apostles, indeed, that Christian courage and magnanimity which they ought to have exerted in that difficult time did fail them; but their faith was nothing so near shipwreck as Peter's faith was. They indeed deserted their Master and fled, Mark 14:50: which they seem to have not done without some connivance from himself, John 18:8. But when Peter renounced and abjured his Lord, how near was he becoming an apostate, and his faith from suffering a total shipwreck? Certainly it was Peter's advantage that Christ prayed for him; but it was not so much for his honour, that he, beyond all others, should stand in need of such a prayer.