The Jews did not faithfully preserve what God told them. Instead they altered it to their own liking, justifying all kinds of disgusting and evil deeds. They were sent prophet after prophet to correct all the errors and lies. One of the first gospels ever written was probably modeled after Deuteronomy, but based on sermons and deeds of Jesus with all the correct teachings, restoring the true law, or rather teaching. It wouldn't last long though, because the early orthodox church did no better than the Jews in refraining from altering everything to suit their agenda. They persecuted everyone who disagreed and destroyed their records.

Even at the very start of our gospels, excluding the fake virgin birth tall tales, the alterations are evident. Take John the Baptist's testimony, where Mark writes

Yōħànàn was proclaiming baptism of repentance for remission of sins. And Judeans and Jerusalemites came to him and were baptized in the Jordan, confessing their sins. And he said to them, "One stronger comes, who I am not worthy of untying his shoes' strap. I baptize you in water, this one will baptize in the holy spirit."

The theme here is John offering a ritual cleansing with water, which is good, but inferior to the coming gift of the spirit. It emphasizes repentance and forgiveness.

But whoever was editing Matthew copied this and changed it to:

(The Name)'s word came to Yōħànàn son of Z'khár'yàh in the desert. And he went to all the place around the Jordan to proclaim immersion of repentance for removing sins, [saying "Repent because (The Name)'s kingdom is close to coming!",] to bring to pass what was said through the prophet Y'shá'yàh, "A voice proclaims, 'In the desert prepare the way of (The Name), straighten a path for our ĕlōhīm in barren land!'". Then they came out to him from Judah and from Jerusalem, confessing their sins and immersing in the Jordan because of his saying. And he saw that many of the Pharisees and Sadduccees came for his immersion and spoke to them saying, "Root of poisonous snakes, who taught you to run away from (The Name)'s flaming nose? Make fruit of repentance, and don't say in your hearts, 'Our father is Áv'ràhàm.' Amen I say to you that the ĕlōhīm is able to raise his son Áv'ràhàm from these stones. And already the axe has reached the root of the tree, and every tree that doesn't make good fruit will be cut off and burned in fire. In truth I am here immersing you in water, but another stronger than me will come, and I am not worthy to untie his shoe's strap; he will immerse you in fire, who has a winnower in his hand to winnow his threshing floor, and he will gather the wheat for his storehouse, but the chaff he will burn in fire that doesn't go out."

Clearly this editor shifted the focus from repentance and forgiveness to punishments for sin and failing to be good enough on your own. Almost all of the revisions shift the teaching in this direction, because the church sought to control people through fear. When Polycarp added this to Luke he made it even worse, having the people specifically ask what works they must do. Jesus's 40 days in the wilderness have also been added to, to "prove" that he cared about the letter of the law and not the spirit of it, and as it was in its current form, by having him appeal to what is written as justification for his actions, in a way unlike he does anywhere else.

And we have Marcion as a witness who caught the church red-handed altering Paul and Luke to their own liking, also watering down faith and grace and mercy for works of the law by the letter, and neutering criticisms of this world, the ways of mankind, nature, and the world order, and later demanding obedience to them. Some of them are completely insane, such as saying that having children is a requirement for salvation (1 Timothy 2:15), even though the very same Paul they are writing in the name of said he believes it is better not to marry at all (1 Corinthians 7:32-35). Unfortunately modern Christianity almost universally sides with the former (although typically not as extreme as thinking that failing to do so means damnation), as with most of these cases. "Turn the other cheek, love your enemy? That's not realistic, let's follow this Old Testament passage 'Love your friend and hate your enemy' instead!". Anything that disagrees with them, or criticizes the world, is immediately labeled "gnostic" and dismissed as heresy, without any honest attempt at refuting it whatsoever. The dissident is then threatened with "hell", a place the church invented based on pagan myths to scare the masses into obeying them, and is the one of the most antithetical ideas imaginable to the message of the gospel. And anyone who dares to look into the matter for themselves is also threatened with hell, and social exclusion.

The entire law is fulfilled with one command: You shall love your neighbor as yourself. How can any of this garbage possibly be reconciled with that? And if were not possible to add to or remove from or change God's word, why does he command against it? And why does he rebuke the lying writings scribes were cooking up in Jeremiah 7-8 which are clearly described as early forms of our written Torah? Not to mention, there is no evidence of God ever canonizing certain books as his own, much less promising to preserve them inerrantly. All of that was invented by people and governments and religious institutions. But the prophets and Jesus and Paul all received their message through divine revelation, and the law written on the heart, not from people. And if even the book itself declares itself to be errenous, how can it be inerrant? It's either accept that this idol is the work of people and not perfect, or stop believing entirely. Otherwise you have no choice but to be a hypocrite, because it contradicts itself. In fact it is easier to "pick and choose" from a book that offers almost everything under the sun, because it is impossible not to, since following one thing means breaking another, rather than simply following the truth.