So... If I'm wrong and Heaven doesn't exist, what have I lost? If I'm right the non-believers who mock the Lord with their ignorant rants and refusal to accept the Christ in their lives.... loose big time.... cause they become only worm food....
I'll take a stab at an answer. Please note, I am not attacking anyone or anyone's beliefs.
For the first question: if you're wrong and there is no afterlife. What you've lost entirely depends on how you've lived.
If you live such that heaven is a reward for being an overall good person, then you're likely to be an overall good person. Even if there is no heaven, you've lost nothing, and the world has gained from your kindness.
If you live believing that the physical world is completely irrelevant due to heaven being the so much more important, then it is likely you will participate in activities that unnecessarily damage the planet or burden those around you. If there is no heaven, the world you left is worse because of you.
These are not the only two options and are not mutually exclusive. Most believers I've interacted with seem to fall somewhere in between the extremes, with their level of care for this life and the afterlife being dependent upon the exact situation in question.
As for the second question: what if the non-believers are wrong. There are again two parts.
First, if the non-believer, in thinking that this world is all there is, chooses to treat it well, then the world benefits from his or her actions. If there is a heaven, the soul will be judged. If it is judged on actions, there should be no problem. If it is judged on faith, then even someone who chose to be a good person will be punished.
However, if the non-believer chooses to treat the world and those in it poorly simply because when he or she is dead, it won't matter, the result is fairly simple. By either judgement basis, acts or faith, the soul would be condemned.
There is one significant wrinkle in all this, though. I used the word non-believer, but a more accurate term would be incorrect-believer. If faith is the basis for judgement, even a Christian would be condemned when they reach the gates of Valhalla. This is essentially Pascal's wager.
The TLDR version is that we should treat the world and those in it well, act as though there is no afterlife, and believe whatever makes you a happier, kinder person.