That thy faith fail not (ina mh ekliph e pisti mou). Second aorist active subjunctive of purpose with ina after edehqhn (I prayed) of ekleipw, old verb. Our word eclipse is this word. Evidently Jesus could not keep Satan from attacking Peter. He had already captured Judas. Did he not repeatedly attack Jesus? But he could and did pray for Peter's faith and his praying won in the end, though Peter stumbled and fell. And do thou (kai su). The words single out Peter sharply. Once thou hast turned again (pote epistrepsa). First aorist active participle of epistrepw, common verb to turn to, to return. But the use of this word implied that Peter would fall though he would come back and "strengthen thy brethren."