(A very brief, probably flawed, intro of the covenant of grace.)
The scriptures teach the idea of one covenant of grace, sometimes referred to as the covenant of redemption or the eternal covenant, with different administrations of this covenant taking place in time. (Titus 1:2; Hebrews 13:20) In this covenant made before time God the Father gives God the Son for “many,” God the Son offers Himself in place of sinners, and God the Holy Spirit administers this covenant in time. No one can receive God the Holy Spirit unless they know Him (John 14:17), those who have not the Spirit of God cannot be saved (Rom. 8:9), if you deny God the Son you do not have God as your Father (1 John 2:23) and without a knowledge of our Triune God you are “without God.” (Eph. 2:12)
This Covenant is “…an everlasting covenant with them, that I will not turn away from them, to do them good; but I will put my fear in their hearts, that they shall not depart from me.” Jer. 32:40 Jesus Christ is our great high priest (Heb. 3:1, 4:14) who prays for us (John 17, 17:9; Rom. 8:43) and lives to do so. The London Baptist Confession of 1689 explains, “This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father, upon the efficacy of the merit and intercession of Jesus Christ and union with him, the oath of God, the abiding of his Spirit, and the seed of God within them, and the nature of the covenant of grace; from all which ariseth also the certainty and infallibility thereof.” (Romans 8:30 Romans 9:11, 16; Romans 5:9, 10; John 14:19; Hebrews 6:17, 18; 1 John 3:9; Jeremiah 32:40)
“Now all this, proves the antiquity of the covenant of grace; nor is it any objection to it, that it is sometimes called the “second” and “new” covenant, Heb 8:7,8,13 9:15 12:24 for it is so called, not with respect to the covenant of works made with Adam, as if it was the second to that, and newer and later than that; for it was made long before that, even in eternity, as has been shown; but the distinctions of “first” and “second”, “old” and “new”, respect the different administrations of the same covenant of grace in time: the first administration of it began immediately after the fall of Adam, and continued under the patriarchs, and under the Mosaical dispensation, unto the coming of Christ; and then a new administration of it took place, which made the first old, and is called the second, with respect to that; and yet both, for substance, are the same covenant, made in eternity, but variously administered in time.
There are several time covenants made with men; as with Adam, Noah, Abraham, the children of Israel, Phinehas, David, etc. But the covenant made with Christ, and the elect in him, was not made in time, but in eternity. It is a notion that commonly obtains, that God makes a covenant of grace with men when they believe, repent, &c. but it is no such thing; the covenant of grace does not then begin to be made, only to be made manifest; it then openly takes place, its blessings are bestowed, its promises applied, its grace is wrought in the hearts of men, when God puts his fear there, gives a new heart, and a new spirit, and puts his own Spirit there, to work faith, repentance, and every other grace; but then the covenant is not new made, but all this is done in virtue and in consequence of the covenant of grace made in eternity, and according to the tenor of that.”