Fjuri: What makes these sins worse then others?
Well, there is a scale-of-sin, and some things don't really count as sin, such as what the OT refers to as, "uninentional sin". Even the NT mentions some issue of grey-area when it talks about making up our own mind about petty issues, such as whether to eat food that was intended for idols. One thing it says is, "sin that does not lead to death",(paraphrase) another thing it says is, "happy is he who does not condemn himself in that thing which he allows". So there is some petty, low-level morality, which is relative. Lying to save someone's life is only a technical lie, for example.
Obviously if you steal from your wife, some money, such as a 5 pence coin, even if she found out she would not care, this is only technical theft.
You're digging into deeper issues now that require a lot more explanation. Suffice to say, if, "Christian" means something, then if someone has a lifestyle or an agenda that contradicts the Christian lifestyle then according to the law of non-contradiction they can't be a Christian.
Imagine if you said to me, "I am a biological female" but you are in fact a biological male. That would contradict what it means to be a female.
It's the same with a Christian. It means something to be a Christian. But atheists largely, seem to endorse that it doesn't really mean anything, because they don't think Christianity or "being a Christian" really exists. Basically they ASSUME their atheism is true, and that it doesn't really mean anything to be a Christian.
But those who are Christian know EVERYTHING the bible says about qualifying to be one, such as, "if you love me, you will obey me" which is what Christ said. He also said, "many will say they knew me but I will say on that day, depart from me, worker of iniquity, for I never knew you".
Can you see what gives the game away in that last quote? Someone's actions is what gives the game away. "You shall know them by their fruit".
It also says later on; "this is the test of knowing Him (God) that you love one another. Everyone who hates his brother is not of God" (paraphrase)
(I paraphrase, and sometimes I paraphrase quite badly, as I have no memory for chapter and verses, but I can look them up if you wish.)
Basically you can't be a Christian if you actively pursue an anti-Christian lifestyle/actions/agendas.
Logically there is the possibility that a Christian can commit a one-time grievous act in a moment of passion, such as murder, perhaps. I say perhaps because the circumstances would have to be extraordinary, perhaps a YOUNG/IMMATURE Christian stands a chance of that because like Paul said of some immature Christians he dealt with; "you are still in the flesh". If you are still in the flesh, and pursuing the things of the flesh such as lust, greed, selfishness, anger, ETC...there is a greater chance. If you are a backslidden Christian that is no longer close to the Lord and is no longer listening to Him each day, there is a greater chance.
The closer you are to God, the less likely it is, because your whole DESIRE is to follow Him. I know that must be hard to understand for someone who does not know God.