Paul 'showed' nothing.
Rome INTERPRETS it this way - keeping it's members in a state of fear of LOSING their salvation if they do NOT get regular and periodic absolution from their priest.
Let's try again:
I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. (Gal. 5:21)Explain how one can be saved by faith alone and still be blocked from the kingdom of God because of sin.