I seem to be writing quite a few articles in response to other people's work at the moment. This is another such, inspired by Bruce Charlton's post about why souls might reject Heaven. Because in our time many do. These individuals would probably not put it like that, or even think of it like that, but it is what a person does and what he is that counts not what he says or even what he thinks with the outer mind. "As a man thinketh in his heart, so is he" as it says in the book of Proverbs. And many people do reject Heaven because they reject the mindset that is required to get there. Like attracts like. You only get to the true heavenly world if there is that in you that corresponds to that world. If you are on some level made of the same stuff, spiritually speaking. In fact, to get to heaven you must already be a subject (it's a kingdom not a state) of that world.
A major factor in the rejection of Heaven is unrepented sin which means identifying yourself with a sin to the extent that you will not or cannot renounce it. You may even no longer accept that it is a sin because it has become part of your sense of self. It has become what you think of as you.
A commenter on Bruce's post asked the question in the title of this one. He went on to answer it himself and said that sin is loving the physical over the spiritual. That is right as far as it goes though I would change the word loving to desiring. But we can unpack this idea of sin a little more and I suggested in a further comment that sin is not loving the physical over the spiritual so much as loving oneself over God or putting oneself before God. That is not to say one should hate oneself, that also is a sin as we are God's creation, but God comes first.
Bruce Charlton defined sin as not being aligned with God's will and divine creative purpose. This sums it up. We are all sinners to some extent because we are all in thrall to the ego. It is the mismatch between ego and God or the human self and the Divine Self that is the root of sin. As long as the human self seeks its fulfilment in something other than the Divine Self it is a sinner and this might even include those who seek absorption in impersonal being. They may be sinners of omission rather than active sinners but they have rejected God as a personal being and so they have rejected love. That is a sin of sorts and will have its consequences, even if these are what the individual in this case wants or thinks he does. So he may get what he wants but it is not the best that God has to offer the soul.
What is sin? Fundamentally, I would say it comes down to one thing and that is not loving God. Therefore you can be as virtuous as you like but if you don't love God you are a sinner. To those of an antinomian disposition who might claim that loving God excuses you of all moral obligations or lapses I would say that "If you love me you keep my commandments". The true love of God automatically makes a person behave in a godly fashion or, at least, aspire to behave like that. He may fall short but he always seeks to stand in the light and to reflect that light as best he can. To claim you love God and carry on disobeying his commandments shows that not only do you not love God, you don't even begin to know him.