Now there were dwelling at Jerusalem Jews, devout men, from every nation under heaven.
These were devout men, careful and circumspect in their devotion to God's commandments, dwelling at Jerusalem either as permanent residents attracted by the temple or as visitors come for the great feasts. They had been scattered into nearly every nation on earth, yet all sought to gather here for worship.
AI summary
These were Jews and proselytes who had left their own countries to come to Jerusalem, some to learn Hebrew for Scripture, others expecting the Messiah according to Daniel's prophecy. They came from the scattered diaspora, the remnant dispersed by ancient captivities, and were religious men with the fear of God before their eyes.
AI summary
there were dwelling at Jerusalem Jews, devout men out of every nation--not, it would seem, permanently settled there (see Act 2:9), though the language seems to imply more than a temporary visit to keep this one feast.