Morality existed for millions of years before religion. It exists because humans are a social species and we need ways to cooperate. Morality doesn't need a justification any more than objects falling downward needs a justification. It comes from the structure of human nature.

What can't be justified from nature is the many arbitrary rules religionists came up with, like treating cows as holy, not eating pork or shellfish, wearing tassels on your clothing or funny hats, etc. etc.