John 6:64, 66-71 But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him. From that time many of his disciples went back, and walked no more with him. Then said Jesus unto the twelve, Will you also go away? Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life. And we believe and are sure that thou art that Christ, the Son of the living God. Jesus answered them, Have not I chosen you twelve, and one of you is a devil? He spake of Judas Iscariot the son of Simon: for he it was that should betray him, being one of the twelve.
Jesus knew from the beginning of His ministry who they were that did not believe in Him not, and who was to betray Him. Jesus knew two things from the beginning of His ministry. Many disciples went back – left Christ and did not follow Him anymore – but not the twelve for He asked: “Will you twelve also abandon Me?”
Peter answered that they had no one else to go to, that Christ alone had the words of eternal life, that Jesus was the Christ and that Christ was the Son of the living God. This kind of confession brings the new birth (1Jn. 5:1)
Christ chose the twelve when they were eager to follow Him and they were seeking to hear God’s will. ‘One of you is a devil’ – Here it reveals Judas as an adversary of Christ and under the influence of a devil (demon) and not as some say that he was satan.
Judas Iscariot was an ordinary man, the son of Simon (John 12:4; 13:2, 26); a genuine chosen and empowered apostle (Matt. 10:1-20; Mark 3:14-19; Luke 6:12-16; 9:1-10; Acts 1:17); the one carrying the purse of the disciples (John 12:4-6; 13:29); and a successful teacher and healer (Mark 6:7-13; Luke 9:10). He was named “Iscariot,” meaning “man of Kerioth,” a place in Judah (Jos. 15:25). He became a “thief” and an “adversary” of Christ late in His ministry (John 6:70; 12:4-6). He betrayed Jesus (Matt. 26:14-16, 47-50; Mark 14:10-11, 43-45; Luke 22:3-6, 47-49; John 13:2; 18:2-5; Acts 1:16-25), returned the money to the chief priests (Matt. 27:3-10), committed suicide and is lost (Matt. 26:24; 27:5; Mark 14:21; Luke 22:22; John 17:12; Acts 1:16-25). There are prophecies concerning him (Matt. 26:21-25; Mark 14:18-21; Luke 22:21-23; John 13:18-26; 17:12; Acts 1:16, 20; Psa. 41:9; 69:25; 109:8; Zech. 11:12-13).