John 10:26-28
26 But ye believe not, because ye are not of my sheep, as I said unto you.
27 My sheep hear my voice, and I know them, and they follow me:
28 And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
The Grace New Testament Commentary - John 10:26-27
10:26-27 Jesus repeats, "you do not believe," and gives the reason: "because you are not of My sheep." While this could be an allusion to election (individual or corporate), more likely the point is that if they were of His flock now, they would believe.
He is not suggesting that his listeners will never become part of His flock, or that they are unable to do so. They simply are not at this point. Nor is He saying that those who believe in Him never stop believing (cf. Luke 8:13). It is clear in this context that these people have never believed in Him.
Some commentators make much of the fact that Jesus says of His sheep, "and they follow Me." They think He is saying that if a person believes in Him, then he perseveres in discipleship. But the context does not support this assertion. Jesus is using figurative language. That sheep hear and follow their shepherd illustrates people believing in Jesus, "the good shepherd" (v 10). Verses 25-26 deal with unbelief and v 27 with belief. Commitment of life and ongoing obedience are completely foreign to the text. Compare John 6:28-29.