This is a very common argument that comes from a misconception of what theists argue. There's a difference between knowing what's good or bad, and doing it. What theists argue is not that only who believes in God can do something good. They argue all human beings know something good or bad because God imbued humans that ability, regardless of what they believe in, or whether they want to do it or not.
Interestingly, Christianity is unique regarding this matter. While most, if not all, religions say you have to earn it in order to go to the desirable afterlife (heaven/paradise/etc.), Christianity says no one, whatever good things they do, can earn it. Only because of God's grace they can be saved. Also, we love because God loved us first. We do good things not for earning an eternal life, but for an act of gratitude that God has saved us.
It sort of comes off to me as a lighter way of asking how you can be a moral person without the threat of eternal suffering in the afterlife (not to suggest that you actually mean that).
I consider myself agnostic, practically an atheist, I just don't really like associating myself with the "hurr durr sky daddy" types, I think religion and the idea of a creator are valuable to help people cope with situations which are totally out of their control, but I can't really relate to needing a creator to give me purpose. I give myself purpose by doing and supporting the things that I feel are right/important.
I don't think existence needs any overarching purpose offered by a creator, it's fine that everything is ultimately meaningless. If anything I find it relieving since it also means that our failures also only have as much meaning as we ascribe to them.