You are mistaken. I disagree that 'powerful Christian' is an oxymoron.
No, you are mistaken. How many times did the Lord or His apostles INSTRUCT people to do something, to "put on" the whole armor of God, to "add to your faith." So if you don't put it on, or add to your faith, your power does nothing. It is not engaged. If all we had to do to be powerful was walk around "exuding power," then 90% of the commandments in the New Testament would be unnecessary.
Again, I know plenty of totally "unpowerful" Christians.