"Q: Why are the old and the new covenants a mystery to many 
Christians?
A: Because most have assumed that the old covenant was God's way of saving people during the Old Testament and the new covenant was God's means of saving people during the New Testament.
This confusion, combined with humanity's natural disposition toward ceremonialism (making the symbol the means rather than the expression of faith), has led to the notion of a covenantal dispensationalism in which the gospel is distorted.
The everlasting covenant is the same as the new covenant. It is the good news of the gospel. Christ crucified is the sinner's representative, Substitute, and Surety. The sinner is unable in his own power to fulfill his obligations regarding the law. Christ, the Surety of the covenant, fulfilled the righteousness of the law on behalf of the sinner, as his representative Substitute. as his representative Substitute.
--The restoration of what man lost is complete, is sure, already in Christ.
The everlasting covenant is the same as the new covenant. It is the good news of the gospel. Christ crucified is the sinner's representative, Substitute, and Surety. The sinner is unable in his own power to fulfill his obligations regarding the law. Christ, the Surety of the covenant, fulfilled the righteousness of the law on behalf of the sinner, as his representative Substitute. as his representative Substitute.
--The restoration of what man lost is complete, is sure, already in Christ.
--Now every man, woman, and child can by faith experience Calvary at Sinai- as the great law of love, the Ten Commandments, is written on their hearts by the One who has fulfilled His everlasting covenant in and through Christ." PaulPenno