Socrates was supposedly calm in the face of his death sentence. When asked why he was so nonchalant, he argued that a fear of death is irrational because either:

• there is no afterlife, which means I won't be around to know what death is actually like
• or there is an afterlife, in which case death is kind of an illusion as you carry on existing

The thing I like about this argument is that it doesn't depend on belief. It works whether you're a theist, a theist who doesn't believe in an afterlife, or even an atheist.

In my opinion, people are scared by death because its almost impossible to imagine yourself not existing. Therefore, when people consider their own death, they kind of imagine a disembodied consciousness in an empty void with nothing but their own thoughts to keep them company.

But this invokes substance dualism, as it says your consciousness could exist independently of your mind. I'm not saying dualism is wrong, but most people who deny dualism still have a vague idea in the back of their heads that something is left over after death.

Ultimately though, its what works for you. If belief in God helps you deal with death anxiety, that's great! Other people, myself included, need a more rational argument that covers all bases like the Socrates thing.