Earlier quoted context omitted.
> ...the mainstream Christian opinion that the Old Testament is vengeful and the New Testament is merciful... That's not mainstream Christian opinion. It's called Marcionism and it's universally condemned by mainstream Christianity.
That's not mainstream Christian opinion. It's fair to say it misses many subtleties of mainstream Christian view, but it is still more true than untrue. The theology of sanctifying grace[1] and the mercy of salvation[2] are both New Testament ideals which are the core of mainstream Christian theology. It's called Marcionism and it's universally condemned by mainstream Christianity. Marcionism is a whole other thing,…
No, "the theology of sanctifying grace and the mercy of salvation" are as old as the hills, and core to both the Old Testament and New Testament. You see hints of God's grace and mercy as early as Genesis 3, often referred to as the Protevangelium, i.e. the first glimpse of the Gospel in the Bible.
The themes of God's perfect justice, powerful salvation, abundant grace and mercy run right the way through the Bible.
That's at least how Moses himself understood the God of the Old Testament, see Exodus 33-34:
"Then the Lord came down in the cloud and stood there with him and proclaimed his name, the Lord. And he passed in front of Moses, proclaiming, “The Lord, the Lord, the compassionate and gracious God, slow to anger, abounding in love and faithfulness, maintaining love to thousands, and forgiving wickedness, rebellion and sin. Yet he does not leave the guilty unpunished; he punishes the children and their children for the sin of the parents to the third and fourth generation.” Moses bowed to the ground at once and worshiped. “Lord,” he said, “if I have found favor in your eyes, then let the Lord go with us. Although this is a stiff-necked people, forgive our wickedness and our sin, and take us as your inheritance.”"
For David's same understanding of the God of the Old Testament, see Psalm 23:
"The Lord is my shepherd... surely your goodness and love will follow me all the days of my life"
And for Ezekiel, see Ezekiel 33:
"‘As surely as I live, declares the Sovereign Lord, I take no pleasure in the death of the wicked, but rather that they turn from their ways and live. Turn! Turn from your evil ways! Why will you die, people of Israel?’ “Yet your people say, ‘The way of the Lord is not just.’ But it is their way that is not just. If a righteous person turns from their righteousness and does evil, they will die for it. And if a wicked person turns away from their wickedness and does what is just and right, they will live by doing so. Yet you Israelites say, ‘The way of the Lord is not just.’ But I will judge each of you according to your own ways.”
Jesus of course is no different in his understanding of the Old Testament with reference to himself and his saving purpose, see Luke 24 and then read Isaiah 53 by way of comparison.
And Jesus' authorized representatives (the apostles) are no different, see 2 Timothy 3:15 where in context Paul is specifically referring to the Old Testament and the ability of the Old Testament to make people "wise for salvation through faith in Christ Jesus":
"...how from infancy you have known the Holy Scriptures, which are able to make you wise for salvation through faith in Christ Jesus."
Regarding your statement that "Marcionism is a whole other thing":
No, Marcion "rejected the Hebrew Bible and the God of Israel. Marcionists believed that the wrathful Hebrew God was a separate and lower entity than the all-forgiving God of the New Testament."
Contrary to Marcion, Jesus' (the Christ's) own view is that the Bible, from the OT to the NT, has one unified message, which is centered in himself and God's saving purpose for men and women hostile to him, which was accomplished through his sacrifice for sin (his death and resurrection), in their place, all of which was promised for centuries beforehand through the Old Testament (see Isaiah 53 again for an accessible example).