We stand before God, the Judge of all.
God is and remains the King and Judge who speaks the last word over all and we must all appear before him to give account (Hebrews 4:13). The all people could meaning the living and the dead, but in connection with the context of the text it is more plausible to think especially of all believers. All those, to which God renders entry into heaven in his final judgment. It is he who opens and closes. As Judge he is and remains the holy God, as Hebrews 12:29 concludes: our God is a consuming fire!
This is how God made himself known at Mount Sinai, and still, he is a fire which consumes those who (continue to) reject him (Psalm 97:3).
23 and to the assembly of the firstborn who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous made perfect,