To John’s surprise, Jesus says: “Do not try to prevent him, for there is no one who will do a powerful work on the basis of my name who will quickly be able to say anything bad about me. For whoever is not against us is for us.

No, at this time the man does not need to accompany Christ in order to be on Jesus’ side. The Christian congregation is yet to be formed, so the fact that the man is not traveling with Jesus does not mean that he is an opposer or promoting a false religion.