Paul is showing how God is glorified in salvation, because the true nature of fallen man that all have sinned and fallen short of the glory of God so we needed someone outside of ourselves. All of us fail to glorify God the way that we should through perfect obedience so none of us could be justified by our works. Instead, it required a Redeemer, because we could never pay the debt. By His grace, He freely justifies us to demonstrate His righteousness and His mercy because He passes over the sins that were previously committed.