“Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;”
King James Version (KJV)
Joh 8:31 If ye continue in my word, [then] ye are my disciples indeed. They "believed on him", but their faith was not yet made perfect by obedience. Hence he adds the condition of discipleship. Their belief must move them to accept his word and obey it. There is a condition, "continue in my word"; a promise, "shall be my disciple".