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.”
Boyd’s Bible Dictionary:
(Isa. 40:11; Zech. 11:4; Matt. 26:31; Luke 12:32; 1 Pet. 5:2-3)[SHEEP.]
Concise Bible Dictionary:
A term used in the O. T. for Israel as sheep gathered by God as their Shepherd, and called Jehovah's flock (Psa. 77:20; Psa. 107:41; Jer. 13:17). It is also applied to those of Israel that were gathered to Christ when on earth. To these He added the Gentile believers; and all were united into one flock (not “one fold”), with Christ as the one Shepherd (John 10:16). When the leaders of Israel were to be judged as not caring for the Lord's flock, the prophet speaks of the remnant as the poor of the flock (Zech. 11:7, 11: Compare Luke 6:20).
The Lord also spoke to His disciples as a little flock, bidding them not to fear: it was their Father's good pleasure to give them the kingdom (Luke 12:32). In Paul's address to the elders of Ephesus he exhorts them to take heed unto all the flock: the wolves would not spare them. Paul commended the shepherds to God and to the word of His grace (Acts 20:28-29: Compare 1 Pet. 5:2-3).