Welcome to CF.
As an aside, you'll meet a wide cross-section of people here with quite a range of opinions on things. You'll find that Christians are not a monolithic group; but there are certain core things that almost all of us hold to. There is just more time devoted to discussing the differences than the common things. You'll also meet many people with stories to tell of their spiritual journey. I personally enjoy reading about people's lives, struggles, and victories.
My answer to the "worst" sin is going to be a bit complicated. Sorry.
One thing to note is that the general Christian concept of sin is also related to the idea of "falling short of the mark" and not just breaking a set list of rules and laws. Many people lose sight of this because it's often easier to recite a list of rules to get excited about than to explain principles. It is the idea that there is a physically, emotionally, and spiritually healthy life for us to be living and sin is missing out on that.
One Bible passage many Christians look to as being definitive of the Christian life is "the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control". The general idea is we are changed into people for whom that is true. It is not so much about avoiding bad things, but becoming people for whom loving God and people is the natural outflow of our lives. It is the idea that patience is about becoming a person who is truly patient inside, not merely taking anger management classes to try to control it. I prefer to focus my attention on the goal and destination, not the departure point.
The central message of Christianity is two fold. I'm going to try to avoid common theological jargon and capture the gist of it. The first side of it is that God forgives our sins through Jesus Christ. The general idea is that people ourselves are a large source of human suffering and pain and evil in the world. None of us is guiltless with respect to that. Some of us are worse than others. But we all need to own up to our contribution to it. The issue is that God is going to eliminate all of the evil in the world, but doesn't want to chuck us out on the garbage heap along with it. The main point of forgiveness is that He is no longer going to hold our contribution to the evils of the world against us. Now that is good, but it doesn't deal with the issue that we are still contributing to it. The second part of the message is that He will work inside of us to change us. It is not about following rules, but about being changed into a new person that naturally follows the rules. It is about being changed into a person that no longer makes the world a worse place but a better one. It is about becoming a person for whom love, joy, peace, patience, kindness, goodness, gentleness, and self-control are a natural description of who we are inside.
In my opinion, the only unforgivable sin is simply explicitly rejecting everything to do with God until your last dying breath. To me, it's saying that "I am not contributing to evil in the world, I need nothing from You. I'm fine exactly the way I am. Go away!" Other sins contain some degree of temporal consequence of various types . For example, if I murder someone, I will probably spend the rest of my life in prison. If I become addicted to heroin, my life will be shortened and I will struggle with severe physical and emotional issues. However, it is only the permanent and total rejection of any spiritual responsibility and remedy that is unforgivable. So, that's what I would consider the worst sin.
If we put weight to sin how do we forgive? Atheist is your claim in the journey so far and this is probably due to rules of Churches vs judgment of self.
