Christians, can you explain to me why God would mandate belief by faith when he could prove his existance now?

Actually God has proven his existence to some people. People who already believe without seeing...many of them God has revealed to in some manner. Why don't you try reading the Bible? Besides, God gave us the choice to believe in him because he wants us to choose him, not force us. If he showed himself, we would automatically believe. But it wouldn't be the same as having faith and believing without seeing. I wouldn't be to skeptical....the consequences aren't very good.