Power and money possess an inherent nature to corrupt people who possess them. That's God's scheme of things. Even when people have faith in God, the nature will still play a role. May be you are right that a believer is less likely to get totally swayed by the evil powers; he is likely to have a better conscience developed -- better values, better checks and balances. May be he will have a better scope for his course-correction if he gets out of bounds with his power and money.