I need a good response for this claim

Someone online said that as a former Christian, one major thing they had a problem with was the fact that Christianity said that you can only be saved by believing, regardless of how much good you do. Now, my first response was that what God says goes and that his word has far greater power than our words but I realized that wouldn't cut it, for this claim is what caused many Christians to turn away from Christianity. Thinking about this has also made me question God's word. Although I would never reject God even if I disagree with him, this is causing a great amount of internal debates and justification as to whether this is true or not. I know many atheists and agnostics who have done more good things than Christians and do feel sorry for any wrongdoing, just not sorry towards God. To me, this seems unfair, and I would like an answer that justifies this or explains it.