Can a Chrisitan lose his/her salvation?

[Lek]

I'm not sure.  The bible doesn't state specifically that your opportunity to be saved ends after death.

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You might as well just believe in universalism, the idea that everyone, atheists included, will be redeemed.  After all, why not?

It's an option, but only after an appropriate time in hell. Actually, there's a good amount of evidence in scripture to support that belief. The individual would still need to come to Christ. I'd refer to it as "universal reconciliation". Some early church fathers supported that idea.