"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."

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).

doesn't change the fact that OT God is a psycho that routinely sanctioned genocide against any fools he disagreed with.

please go ahead and refute any of the following crimes against humanity which this psycho happily documented for all time.

https://rationalwiki.org/wiki/Examples_of_God_personally_kil...

Another example in evangelical Christianity is the "new covenant vs old covenant" theology.

The difference between New Covenant and Old Covenant is a matter of a change in place, not a matter of a change in God's character (which stays the same).

The Old Covenant was the law written on tablets of stone, see Exodus 33-34.

The New Covenant is the law written on tablets of human hearts, see Ezekiel 36.

The place changes from "tablets of stone" to "tablets of human hearts".

It's also critical to understand that both covenants were given to those who were ALREADY God's people, saved on the basis of grace, through faith. The Old Covenant was given to those who had already been saved out of Egypt by God's powerful grace. The New Covenant is written on the hearts of those who are saved through faith in Christ's suffering, death and resurrection on their behalf. The Exodus then, rightly understood, is a foreshadowing of the need to be saved from slavery to sin (Jesus' own commentary in John 8).

Salvation in both OT and NT has always been on the basis of faith, see Abraham ("and his faith was counted to him as righteousness" - and this long before the Law was given). Salvation has never been on the basis of Law-keeping since the Law was given to fallen man after the events of Genesis 3 (Romans 5). We are not sinners because we break the Law. We break the Law because we are sinners. The Law makes people conscious of sin, and drives them to the grace of God. Once there, the Law shows people how to be godly, because the Law reflects God's own character.

So the difference is where the Law is written, not the contents of the Law itself. As Jesus said, the Law is to "love the Lord with all your heart, soul, mind and strength and to love your neighbor as yourself".

But knowing that, or trying to keep the Law externally will only lead to a knowledge of sin. It's faith in Christ alone which saves. We are not saved BY works, but FOR works and then again only once we are saved (Ephesians 2), and that by grace alone, through faith alone, in Jesus Christ alone, according to God's words alone, for God's glory alone.

I'm referring to this theology more as a cultural and mythological phenomenon, which is why I specified "evangelical". If you step outside of the text for a minute and look at how these things have gotten unpacked, maybe you will catch what I mean.

Many people interpret the new covenant as God's change of heart. I don't think it is internally possible to acknowledge that the new testament and old testament gods are "different", for that would be a surface violation of monotheism, which is the common understanding of Moses' "first commandment". (this is problematic on many levels, but its the state of popular Christianity). To many Christians, Moses' first commandment is still wrapped up in "put no god before me", which they interpret to mean that Buddhists and Muslims can't "get saved" or some other similar thing. Ugh. The point being that its not safe for some fearful Christians to acknowledge that in their own belief system, "god" takes radically different characters in different contexts... That's too much cognitive dissonance and flirts with a violation of monotheism. The same cognitive dissonance hovers around trinitarian theology, which people also idolize.

Whereas in popular practice, there are myths saying God changes his basic agreement with humankind, and that it represents a change of heart. That God has a moment.

Even if theologically the idea of the old and new covenants is that human beings have come to a fresh relationship with god through some realization and change in their hearts, in evangelicalism it is too commonly is unpacked as "God used to be angry, but now he forgives us. Specifically because of Jesus sacrifice, we get a 'second chance.' Don't blow it."

I understand what your saying too, and find it compelling. I just don't wanna ignore the cultural myths attached to these theologies. Those are what make it a religion... how people unpack it.

All that said, I really do like your interpretation of the kernel of these theologies.

I'd also point out that Jesus says "I came to fill up the law". Jesus emphasized the positive character of the law, whereas Moses' was prohibitive, framed in negative terms. A lot of messianic type folks miss that one. They interpret this as a statement of pending apocalypse.

But it's really just more of a throwback to the original grace, and simple righteousness, of an integrated passion for your life and the people in it.

"What does the lord require of you but to do justice, love mercy, and walk humbly with your god?" says Micah as he forecasts that some of the the poorest, marginalized people will realize, in a major way, that the answer to that question is "not much else".