Bruce blames God for his failures but when receiving God's powers, he turns out to be a selfish hypocrite using his powers for his own gain and ignoring the responsibility of God. Well deep down he's a good guy who was just at his lowest in his life who admits his mistake and repented. One of the things i like about movies is character growth.
I remember Morgan Freeman being a controversial pick for God.... but damn does he nail it. I have a hard time thinking of anyone else that could have pulled it off. Anyone else would be too serious for comedy or too silly to take seriously. Morgan Freeman just nails that narrow range between being easy to take seriously but still able to get humour through.