Christ fulfills His office in, according do, with, and through both natures...

In fulfilling Christ's office, the person does not act and work in, with, through, or according to only one nature.  It works in, according to, with, and through both natures.  As the Council of Chalcedon expresses it, one nature works in communion with the other what is a property of each.  Therefore, Christ is our Mediator, Redeemer, King, High Priest, Head, Shepherd, and so on, not according to one nature only (whether it be the divine or the human), but according to both natures.
~BOC, FSD, VIII, 46-47

Forgiveness comes through grace, without any work, merit, or worthiness of ours preceding, present, or following...

We believe, teach, and confess what our righteousness before God is this:  God forgives our sins out of pure grace, without any work, merit, or worthiness of ours preceding, present, or following.  He presents and credits to us the righteousness of Christ's obedience.  Because of this righteousness, we are received into grace by God and regarded as righteous.

We believe, teach, and confess that faith alone is the means and instrument through which we lay hold of Christ.  So in Christ we lay hold of that righteousness that benefits us before God, for whose sake this faith is credited to us for righteousness.

We believe, teach and confess that this faith is not a bare knowledge of Christ' history, but it is God's gift.  By this gift we come to the right knowledge of Christ as our Redeemer in the Word of the Gospel. And we trust in Him that for the sake of His obedience alone we have--by grace--the forgiveness of sins and are regarded as holy and righteous before God the Father and are eternally saved.
~BOC, FE, III, 4-6