ones who thinks they can live however they want and then say the magic words to go to heaven after a life of being a shitty person.

I would actually argue that's not Christianity, because the Bible never talks about it. Yes, repentance exists, but if you were actually truly repentant, then you would try not to be a disgusting person. There aren't any specific magic words either, you can say what you want to others, but God knows your heart.

I think the origin of this comes from Luther's idea of Sola Fide - justification by faith alone. However, this was moreso made as a response to the Roman Catholic idea of venial sins, mortal sins, works based salvation and legalism in general. He didn't mean literally faith alone, but he meant that if you had faith and followed Jesus' commandments with a true and repentant heart, you would have assurance of heaven, and not need to worry about accidental unconfessed sins or purgatory. As St James writes in his epistle:

‭James 1:26-27 ESV‬

If anyone thinks he is religious and does not bridle his tongue but deceives his heart, this person’s religion is worthless. Religion that is pure and undefiled before God the Father is this: to visit orphans and widows in their affliction, and to keep oneself unstained from the world.

‭James 2:18-19 ESV‬

But someone will say, “You have faith and I have works.” Show me your faith apart from your works, and I will show you my faith by my works. You believe that God is one; you do well. Even the demons believe—and shudder!

As for arguments for God (not counting the whole Jesus thing as well)

Infinite numbers exist, infinity exists. For example, pi has an infinite amount of numbers following it that are preset in the laws of the universe. But the universe is finite, so such concepts can only exist in the mind of an infinite, ie, God.

Morality - We know the difference between right and wrong. For some reason, we think stealing from someone else is wrong even though it would make us better our survival. Or torturing a puppy. People who become ruthless have to override a moral instinct first to do that. If there is no God, then morality is subjective and what we say or do isn't right or wrong. I am not saying that atheists aren't moral, because they are as morality is naturally implanted in us by God.

Fine tuning - The chances of the universe existing as it is was so indescribably small, that I find it hard to believe this all came across by chance. I do believe in evolution, I'm not a young earth creationist. But I believe the whole process was designed to play out as it did.

Depravity - The Bible constantly talks about people falling into depravity and sin, and this is a pattern we see in the real world. We have people destroying the planet for profit and wars going on in Palestine where civilians are just murdered. Also past atrocities like the holocaust. I can see why God is angry at us and will judge us.