There are many who do not believe in God. There are many who believe that Jesus existed but that he is not, in any way, God. There are others who have some very unorthodox ideas about Christ, but we will leave them to one side for now. Basically, the main difference is between those who believe in Him as an historical person - but not God, and those who believe that He is divine: the Second Person of the Blessed Trinity made man.
The more I see of atheists arguments and replies, the more I see people who are choosing NOT to believe. Faith is a gift, but I do not believe this gift is withheld from anybody (what would be the purpose of that?), rather some people, at one level or another, are rejecting faith. I am not saying this is always entirely their fault. The environment in which they were brought up has a part to play, as do other things like personal trauma, illness, lack of education, simple ignorance. Some people seemingly reject faith for what seems to them to be good reasons. God will judge our consciences. I do believe that faith is being rejected, however much that rejection is blameworthy or honestly felt. People ask for proof. I believe proof has been given, but the proof is rejected because it is not the right kind of proof. All proof of any kind also requires choice. I see proof, but I must accept it as proof. There are various kinds of "proof" that are disputed. There are supposedly even people who still maintain that the earth is flat! There are, as we know, holocaust deniers (they dispute historical and documentary proof on the basis that it has been falsified). In the end, with respect to God, there is no absolute, commanding, inescapable proof on this earth. Faith always involves choice - there is no getting around it. This means that faith requires courage because it means taking a risk. Is it more courageous to believe in God (specifically the God of Abraham, Isaac and Jacob)? Yes, because it means change - moral challenge - and commitment. Choosing not to believe is easier because you don't have to worry about being challenged except by circumstances and other people - but then so much of those can be circumvented, avoided or just ignored. Some, of course, cannot, but at least there is no ultimate judgement - no ultimate challenge - no eternal choices. Faith can be a "crutch" but if that's all it is, it is not true faith. Faith is a journey, and it is one that demands a decision and a frequent renewal of that decision in the face of sometimes enormous difficulties and obstacles. At the same time, it is not a journey we take alone, and the One who walks with us is, at any time, capable of lifting us off our feet.