It’s theirs even before they die, but they can also leave it before they die. If they choose to embrace the Lord in preference to it.
We have two former athiests in our church now. You would have never known they were both former athiests though. Both have said basically the same thing. They always suspected that there was a higher power from time to time but claimed to be athiests because of some form of pain in their life that was tied to faux Christians whether it was from hypocrisy they witnessed or were told about in the news media. It wasn’t until they actually visited a church where the Christians and their love were genuine that they felt the Holy Spiit and then understood what it meant from feeling it themselves. The laws of God are written in men’s hearts. Even athiests know this but won’t admit until they understand what that means.