Charlie Harper was a sinner and he was aware of that, enjoyed it even. Since he had the idea that he would not qualify for heaven he should enjoy his life as much as he could.Surprisingly it didn't take much for Castiel to get him to agree to accept the burden and privilege of becoming an angels vessel. Just a simple question and Castiel took his new body and did gods work. And if the human had been drunk at the time, as vessel he was guaranteed a trip to heaven. Castiel decided early not to visit Charlies heaven.