Expositor's Bible Commentary Commentary John 6:64

"But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who it was that should betray him." — John 6:64 (ASV)

Jesus’ reference to those who did not believe is explained by his later allusion to Judas (v.70). Jesus had given ample opportunity for faith to all those who followed him; yet from the beginning his spiritual discernment made him aware of those whose faith was genuine and those whose attachment was only superficial.