John 6:70

It might appear that the Twelve had chosen Jesus as their rabbi, but really the choice had been His (Mark 3:13-19; Luke 6:12-16). He had chosen them and they had then believed on Him even as the Father chose the elect who then believed on Jesus. Reflecting His knowledge of those who believed in Him and those who did not (John 6:64), Jesus revealed that even among the Twelve there was one unbeliever. Jesus had chosen him to be one of the Twelve, but God had not chosen Him for salvation.

The Greek word translated "devil" (Gr. diabolos) does not have an article with it in many reliable ancient Greek manuscripts. This usually indicates an emphasis on the quality of the noun. Here it probably means that one of the Twelve was devil-like (cf. Mark 8:33). The Greek word is the equivalent of the Hebrew satan, meaning adversary or accuser. It means slanderer or false accuser, but when it occurs as a substantive it means Satan (e.g., John 8:44; John 13:2; cf. John 13:27). Jesus probably meant that one of the Twelve was going to behave as Satan because Satan would direct him.