John Calvin Commentary John 18:27

"Peter therefore denied again: and straightway the cock crew." — John 18:27 (ASV)

Immediately the cock crew. The Evangelist mentions the crowing of the cock, in order to inform us that Peter was warned by God at the very time; and for this reason the other Evangelists tell us that he then remembered the words of the Lord, (Matthew 26:75; Mark 14:72), though Luke relates that the mere crowing of the cock did not produce any effect on Peter, until Christ looked at him, (Luke 22:61).

Thus, when any person has once begun to fall through the suggestions of Satan, no voice, no sign, no warning, will bring him back, until the Lord Himself casts His eyes upon him.