Isa 42:6-7
6 I the Lord have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles;
7 To open the blind eyes, to bring out the prisoners from the prison, and them that sit in darkness out of the prison house.
Christ's Covenental Death, for all the Covenant People, which are all those the Father gave Him Per Jn 17:2
2 As thou hast given him power over all flesh, that he should give eternal life to as many as thou hast given him.
And to these it says "and give thee for a covenant of the people,"
These are also the Children of Promise Rom 9:8, now to them, He delivers them all from the prison of unbelief, for that is the promise here indicated in this scripture, because unbelief is a prison, and all for whom Christ died must be brought out of that Prison.
Unbelief is a Prison we know from Paul's statement regarding God's Judgment against the Elect for sin, He concluded them all in Unbelief Rom 11:32
32 For God hath concluded them all in or into unbelief, that he might have mercy upon all.
The key here is the meaning of the word concluded ! It is the greek word sygkleiō and means:
to shut up together, enclose
a) of a shoal of fishes in a net
2) to shut up on all sides, shut up completely
Notice, to shut up on all sides or to enclose !
It means:
to shut or hem in; close in on all sides
Of course the idea conveyed is to confine or to imprison, and so it is , all the elect are by nature imprisoned, confined, enclosed in unbelief.
God is the primary and first cause of this as stated in Rom 11:32, but God also employs satan as a secondary cause, the god and prince of this world, and so it is written of him 2 Cor 4:3-4a
3 But if our gospel be hid, it is hid to them that are lost:
4 In whom the god of this world hath blinded the minds of them which believe not,
All by nature are shut into unbelief by satan and the blindness of the mind he causes by the permission or will of God See Eph 4:18 also !
However, everyone Christ died for, His Covenental and Redemptive Death, by His Blood, has opened blind eyes and brought out the prisoners from the prison of unbelief !
The word prison in Isa 42:7 is the hebrew word macger and means:
a shutting up, locksmith, smith, dungeon, enclosure, builder of bulwarks
Christ's Redeeming Covenental Death for His People brings all for whom He died out of the prison, the enclosure of unbelief which they are in by nature.
So if any that Christ died for remain in unbelief, and never come to saving Faith in Christ, that would mean that satan, the god of this world, was victorious over Christ and His Blood, and God's justice being satisfied for that one did not release them from the prison of unbelief ! That means that satan was stronger than Christ; And many right today, who call themselves Gospel Preachers, they would tell us that a great many for whom Christ died, will died or remain in unbelief.
It should go without saying, that all such preachers and professing christians are liars and are ignorant to the Accomplishments of Christ's Covenental Death !
Zech 9:11
As for thee also, by the blood of thy covenant I have sent forth thy prisoners out of the pit wherein is no water.