The strangers. These were members of the tribes constituting the northern kingdom. During the reign of Rehoboam a similar influx of citizens of Israel into Judah took place (ch. 11:16).
Out of Simeon. Although Simeon lay within the boundaries of the southern kingdom (Joshua 19:1), many members of this tribe probably took their abode within the territory of Israel at the time of the division.
When they saw. When many of the people of Israel observed that God was with Asa and was blessing him, large numbers came down from the northern kingdom henceforth to live in Judah.