For all have sinned and fall short of the glory of God.
Again, we see the word πάντες show up, further increasing the sense of equality among Jew and Gentile alike. To Paul, all are equally guilty of sin, and yet because of this, all need access to salvation. It is clear from 1:18 to 3:20 that not a single person is righteous and Romans 3:23 is a summary statement for that entire section borrowed in large part from Isaiah 59. Paul stresses that it is not our righteousness that saves us as we do not possess any, rather like David says in Psalms 143:11, it is by God’s righteousness that we gain salvation. Brendan Byrne points out that there is a faint allusion to Genesis 1:26-28 in ὑστεροῦνται τῆς δόξης τοῦ θεοῦ which talks about humanity being made in the image of God. Where once we were creatures which could fully reflect the glory of God because of our being made in His image and likeness, so now because of sin do we lack that same glory.
We are only simple mirrors. When we turn away from God, we can not reflect his glory. When we face the darkness, we are darkened ourselves. Mirrors which cannot reflect are utterly worthless as mirrors, and yet God uses us anyway.