You can volunteer without Christianity, but better.
Voluntarism is far better without religion because there is absolutely no incentive or ulterior motive behind it (like "going to heaven" or improve your social credit), it is purely based on selfish desire to do something that you want to do.
In fact, selfishness is the only honest way of life, because you ask for absolutely nothing in return. Other people may call it "helping others", but for us it is just selfish desire to do something that just happen to improve the situation for others.
Who is John Galt?