"Tell me, ye that desire to be under the law, do ye not hear the law?
"For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Hagar.
"For this Hagar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. But Jerusalem which is above is free, which is the mother of us all."
In his commentary on the epistle lesson for the Fourth Sunday in Lent, Melville Scott writes:
"By an allegory taken from the two sons of Abraham, each of whom stood in a different relation to his father, Christians are taught their happy relation to God as His children by grace, and their consequent duty...
"Ishmael owed his slavery to his mother Hagar, the slave-wife of Abraham, a type of the Jewish Church, 'the Jerusalem that now is'--
(1) As wandering in the dreary desert of the Law;
(2) And as only able to give her children a share of her own bondage.
"To the Jews the service of God was a burden grievous to be borne, for God was to them more of a Master than a Heavenly Father...
"Isaac owed his happy freedom to his mother, Sarah, the chosen and beloved wife of Abraham. Sarah, the free mother, whose children are born free, is a figure of the Christian Church: 'the Jerusalem which is above, which is the mother of us all.' As in both cases the position of the mother determined the relation of the child to the father, so the very fact that we belong to the Christian Church by our baptism determines our relation to God and makes us His children. It is because we are members of Christ that we are the children of God and have received the 'one baptism for the remission of sins.' The word covenant practically means relation, or position, and a covenant of grace a relation or position of grace. The word conveys no idea of mutual bargain or condition in either case. We are what God has made us.
"Is the service of sons, which is more than that of slaves. The son submits his will to his father, the slave only his outward conduct to his master. The son gives what the slave withholds, and for this reason we are tempted to prefer slavery; but if he gives more he receives more, for if he must say, 'I am my father's,' he can also say, 'My father is mine.' Hence comes the conscious dignity and liberty, the power of confident prayer and thankful praise. The son serves not for favour but as one in favour, not for wages but as a debtor to infinite love. While the thought of the crown before him gives him joy, his true motive is the Cross behind him."
In the state of grace, the Christian no longer lives under the condemnation of Law. So, when a Christian seeks to truly obey the Lord he should never do so to meritoriously obtain divine favor. He already has God's favor, given freely in Jesus.
In the state of grace, the Christian's works of obedience are never evaluated in terms of strict merit, i.e., how they stand on their own, but with respect to the undergirding merits of Christ and the operation of the Spirit working together with his faith. Our Father does not judge acts of Christian obedience as sinful and neither should we. This is to look at Christian obedience in terms of the world as a closed system, alienated from God and the grace of Christ.
Those who continually revert to conceiving their obedience as worthless servitude betray that they lack the Spirit of Sonship.
When Scott says, "The word [covenant] conveys no idea of mutual bargain or condition in either case," this should be understood in the sense that our covenant relationship is not conditioned upon our own meritorious performance as mere servants earning a wage. God, himself, has provided a perfect and sufficient sacrifice. Out of gratitude for this provision we are constrained to obey God, not as servants but as sons, as Scott says, "debtor[s] to infinite love."
Here, I must warn my Reformed Christian brethren that liturgies that situate believers existentially in the place of Ishmael every Sunday morning are doing a disservice to the people of God.
The spiritual cycle of guilt, grace & gratitude may be a good account of Christian experience as far as it goes, but once redeemed, the normal guilt the Christian incurs is a sin against God's fatherly love, not against pure divine justice. The response of gratitude does not and cannot directly lead into a renewed condition of guilt. True gratitude paves the way from grace toward greater grace.
The full weight of God's naked justice is only experienced by those who stand outside the covenant. If your regular experience as a professing Christian is a continual return to the state of being under the wrath of God, you have fallen from grace and need to be restored through the Sacrament of Penance.
It is plain that a real distinction between venial and mortal sin exists. Venial sin temporarily hinders the communication of grace, but mortal sin leads to despair and separation from our heavenly Father.
Penance is essential in order to convert natural attrition (sorrow for sin's consequences) into contrition (sorrow at displeasing the Father). It accomplishes this this by restoring the fallen to the covenant, in that the penitent passes from the state of judgment to the state of forgiveness--the state of being justified--from a state of alienation to a state of communion.
The sacerdotal system of the Catholic Church therefore represents not a return to Sinai, to a fearful ministry of pure Law, but is in fact the necessary means by which the forgiveness and grace of God comes to his sons.