Hi GandalfTheWhite,
Think of it this way, just as One man without sin paid the penalty for all sin, in the same way that same one man met the righteous requirements of the law on our behalf, satisfying God's requirement for His need for the fulfillment of the law. We are free in Christ! Because we are having faith in Christ, the Father sees us through him as perfect, no sin and the law fulfilled. This goes right along with believers being credited with righteousness. Whatever God sees in Christ, he sees in us, even though we are still sinners. When we put ourselves under the law to meet its requirements, we are not trusting in Christ, but in our own efforts to obtain righteousness.