If there is no afterlife/god then we have to do our very best with this life. If this life is a preparation for an afterlife, then we had better do our very best with this life as well. No difference. The only time there is a difference, is if, there should be a god who propagates and enforces arbitrary rules, which can not be discerned from moral instincts by way of reason.

And if that is the case, then the chance that anyone has won the millions to one lottery of picking the right god and the right sect, is, for all practical purposes zero. As a sceptic, should you be in the wrong, and meet god, then you can honestly say, I did my best and I did not try to set up a fake in your place.

This is the best argument against all religions not just Christianity and there is no good answer. Without they try to explain why god should be unfair.