I said we do not know for sure, as my first line in my response said. God knows the heart of Stephen and it's possible he did all his life's work to truly show love to others. It's possible he sacrificed his lifelong pain for the good of others also.Dr. Watson said:
So a brilliant person who contributed to society and suffered immensely on earth is now in Hell because of his religion. The older I get and the more of the world and it's suffering I see, the more abhorrent I find a lot of these beliefs.
If Stephen was consistently misled about the true nature of God his entire life despite genuinely searching the truth about Him out, then I can understand his decision to turn to atheism (although I don't justify it).
The key question though is if he knew the truth of Jesus and still rejected it, because if so then he's in trouble despite the good that he did.
A vast majority of those that reject God simply do not understand the truth of God. They have a misinformed, indifferent, or twisted version of who God is, so they cannot be held fully accountable for that. Those that do not know God through no fault of their own, but still live a life of true love towards others are still responding to the love God has put inside them. The problem is when people know full well the nature of God, and still live their life in continual rejection of Him.