Origin and Application:
When it was first said in Mark 14:38, the weakness refers to our spiritual weakness as well as physical weakness.
Peter, a disciple of Jesus, fell asleep when he was told to keep watch during the time Jesus was praying just before his arrest. When Jesus returned and saw that Peter did not stay up to keep watch in the night, Jesus said, “Watch and pray so that you will not fall into temptation. The spirit is willing, but the flesh is weak.”
Indeed human flesh is weak. Days later when Jesus was arrested, Peter denied three times that he knew Jesus in order to escape from the Roman soldiers.