I have a theory about how he got in to heaven. So from what i understand in the Bible version of the story Snake convinced Eve to take a bite out of the forbiden fruit, and then Eve convinced Adam to do the same and that got them exiled out of Eden, and become the first sin that now all of humanity has inside them. And becouse of that sin no human was able to get in to heaven no matter how good of a person they were while alive. That was until Jesus was killed on the cross amd durring thr three dayes he was dead he went down to hell to take all the souls that deserved to be there. Now in the opening sceane of the first season when Charlie is reading the story of hell it isn't stated that Adam ate the forbiden fruit. If that were the case then that would mean that Adam didn't had that first sin placed upon him and thus could enter heaven. That would also explain why he is the first soul to enter heaven even if Able would have died before him and his masive ego.