John 6:67

From this time, say Euthymius and others: otherwise the Syriac, on account of this discourse: Arabic, because of this, left Jesus, &c. These disciples were not the Apostles, for Christ excepts them in the following verse. Neither were they the seventy-two disciples. For those had not yet been designated and chosen by Christ. But they were His more constant hearers and followers, "who," as Theophylact says, "followed Him in the rank of His disciples, and remained with Him longer than the multitudes, and so, compared with the rest of the crowd, were called His disciples. These persons therefore up to this time being allured by the sweet doctrine of Christ, fed by the loaves miraculously multiplied, and hoping to be fed in future by similar food, when they heard Christ substituting His own Flesh in the place of bread, and willing that they should eat It, thought either that He was mad, or else was contriving some horrible and savage scheme, or perchance a conspiracy against the Romans, and would inaugurate it by their tasting His flesh and blood, as Cataline had done before at Rome. Thus, to provide for their own safety, they fell away from Christ.

S. Epiphanius declares expressly that one of these was S. Mark, who was afterwards brought back by S. Peter, and became an Evangelist (Hæres. 51): but others deny this, and assert that S. Mark neither saw nor heard Christ (in the flesh), but was converted by S. Peter after His death. So S. Jerome on Ecclesiastical Writers, and others.