Peter said to him, "Even though they all fall away, I will not." And Jesus said to him, "Truly, I tell you, this very night, before the rooster crows twice, you will deny me three times." 
Mark 14:29-30 ESV

Having arrested Him, they led Him and brought Him into the high priest's house. But Peter followed at a distance. Now when they had kindled a fire in the midst of the courtyard and sat down together, Peter sat among them. And a certain servant girl, seeing him as he sat by the fire, looked intently at him and said, "This man was also with Him." But he denied Him, saying, "Woman, I do not know Him." And after a little while another saw him and said, "You also are of them." But Peter said, "Man, I am not!" Then after about an hour had passed, another confidently affirmed, saying, "Surely this fellow also was with Him, for he is a Galilean." But Peter said, "Man, I do not know what you are saying!" Immediately, while he was still speaking, the rooster crowed. And the Lord turned and looked at Peter. Then Peter remembered the word of the Lord, how He had said to him, "Before the rooster crows, you will deny Me three times." So Peter went out and wept bitterly.

JC Ryle writes: "We learn, from these verses — what a fearful enemy the devil is to believers."  He goes on: The personality, activity, and power of the devil are not sufficiently thought of by Christians. Satan is the one who brought sin into the world at the beginning, by tempting Eve. Satan is he who is described in the book of Job as "going to and fro in the earth, and walking up and down in it." Satan is he whom our Lord calls "the prince of this world," a "murderer," and a "liar." Satan is he whom Peter compares to a "roaring lion, seeking whom he may devour." Satan is he whom John speaks of as "the accuser of the brethren." Satan is he who is ever working evil in the churches, catching away "good seed" from the hearts of hearers, sowing tares amid the wheat, stirring up persecutions, suggesting false doctrines, and fomenting divisions.