Adds nothing to Justification before God !
Faith, or believing, the act of the creature [old or new] adds nothing to Justification before God, that fact was accomplished solely by Christ's Blood, however it does give Joy and Comforth in the Knowledge of the fact, Paul's meaning here Rom 15:13
Now the God of hope fill you with all joy and peace in believing, that ye may abound in hope, through the power of the Holy Ghost.
Note: Believing is through the Power of the Holy Ghost !
Yet Justification is a Divine Fact before we believe it, Faith causes us to believe the Fact of our free Justification, as it is made known in the Gospel Rom 1:17. Justification is our Righteousness, and is made known to the Spiritual eye of Faith, and so the Gospel is also called the word of Faith Rom 10:8,17 !