Mercy is an attribute of God, an infinite and inexhaustible energy within the divine nature which disposes God to be actively compassionate. Both the Old and the New Testaments proclaim the mercy of God, but the Old has more than four times as much to say about it as the New.
We should banish from our minds forever the common but erroneous notion that justice and judgment characterize the God of Israel, while mercy and grace belong to the Lord of the Church. Actually there is in principle no difference between the Old Testament and the New.
In the New Testament Scriptures there is a fuller development of redemptive truth, but one God speaks in both dispensations, and what He speaks agrees with what He is. Wherever and whenever God appears to men, He acts like Himself. Whether in the Garden of Eden or the Garden of Gethsemane, God is merciful as well as just.
He has always dealt in mercy with mankind and will always deal in justice when His mercy is despised. Thus He did in antediluvian times; thus when Christ walked among men; thus He is doing today and will continue always to do for no other reason than that He is God. If we could remember that the divine mercy is not a temporary mood but an attribute of God’s eternal being, we would no longer fear that it will someday cease to be.
Mercy never began to be, but from eternity was; so it will never cease to be. It will never be more since it is itself infinite; and it will never be less because the infinite cannot suffer diminution. Nothing that has occurred or will occur in heaven or earth or hell can change the tender mercies of our God. Forever His mercy stands, a boundless, overwhelming immensity of divine pity and compassion.
Tozer, A. W. Knowledge of the Holy: The Attributes of God.