John 10:16
“For other sheep also I have, which are not of this fold, them also must I bring.”
Observe again, the word “must,” here used, does not express necessity, but is declaratory of something which will certainly come to pass. As though He had said, “Why marvel ye if these shall follow Me, and if My sheep shall hear My voice? When you shall see others also following Me and hearing My voice, then shall you be astonished more.” And be not confounded when you hear Him say, “which are not of this fold”, for the difference relates to the Law only, as also Paul says, “Neither circumcision avails anything, nor uncircumcision.”
“Them also must I bring.” He shows that both these and those were scattered and mixed, and without shepherds, because the good Shepherd had not yet come. Then He proclaims beforehand their future union, that,
“They shall be one fold.”
Which same thing also Paul declared, saying, “For to make in Himself of two one new man.”
Source: Homilies on the Gospel of John (New Advent)