17But the mercy of the LORD is from everlasting to everlasting upon them that fear him, and his righteousness unto children's children;
I do not think it will be argued that Justification by Faith is a very great mercy to experience in this life. This mercy was not just random or by chance , or by freewill, but was established as a sure mercy from everlasting. God, the everlasting God, Preached the gospel of Justification to Abraham centuries ago Gal 3:8
8And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
which is referring to Gen 12:3
3And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
But now what was the previous bases for this Mercy ? I believe it was the everlasting Covenant which is what Ps 103:17 is referring to, as the Mercy from everlasting.
I believe Jesus coming in the incarnation was a effect of that Covenant and why He is called the promised mercy.
67And his father Zacharias was filled with the Holy Ghost, and prophesied, saying,
68Blessed be the Lord God of Israel; for he hath visited and redeemed his people,
69And hath raised up an horn of salvation for us in the house of his servant David;
70As he spake by the mouth of his holy prophets, which have been since the world began:
71That we should be saved from our enemies, and from the hand of all that hate us;
72To perform the mercy promised to our fathers, and to remember his holy covenant;
73The oath which he sware to our father Abraham,
77To give knowledge of salvation unto his people by the remission of their sins,
78Through the tender mercy of our God; whereby the dayspring from on high hath visited us, !
So Christ coming to die for His People is the fruition of the everlasting covenant. His coming was an act of mercy, in that He would give himself, his soul an offering for sin, for the transgressions of God's People.
Justification is not something that just happens.
Before Abraham was justified he had to believe God. "Abraham believed God and it was counted unto him for righteousness" Romans 4:3.
Abraham believed that God was going to provide him with a savior. In the Old Testament they were justified by faith in God's promise. Today we are justified the same way, BY FAITH.