Dictionary of Biblical Words:
The fold in John 10 means the Jewish enclosure formed by God’s law and ordinances, out of which the rejected Christ was then leading His sheep, above all, in His death; after which He would form one flock, composed of believing Jews and Gentiles, His “other Sheep which were not of this fold” or Jewish people (ver. 16). One fold is wrong; it should unquestionably be rendered “one flock” as is now well known. There is no longer anything recognized of God as one fold (as Romanism). still less, many folds as Protestantism, but one flock, one shepherd, or as the Apostle puta it, “One body and one Spirit.”
Concise Bible Dictionary:
Sheepfold
The divinely appointed system of Jewish ordinances which formed the enclosure into which the Lord entered by the door, in order to find His own sheep and lead them out. Gentile believers were added to them, and they became one flock (not “one fold”) with one Shepherd, the Lord Himself (John 10:1, 3, 16). There is no longer a fold on earth for those that are Christ's. They are formed into the church, namely, the one flock.
Sheepfold—Taken at Beit Sahur, December 4, 1937