If my son needs the car, I can give him my credit card to use at the pump. At his current economy, he usually doesn’t have the means to get what he needs. So he uses the card with my name on it. With my permission and according to my desire, he assumes my identity. Though he cannot fulfill the conditions required for payment, my son has all of my credit available to him. He meets the qualifications requires to use the pump b/c the machine gives him the credit that really is mine. My son, though he could not provide it himself, acts with my identity and, thus, has all the credit that I have earned.
This is what it means to be “in Christ.” Though we cannot provide the perfect righteousness ourselves, in Christ we can act with His identity and have all the credit that He earned for us.