2 Chronicles 15:9

And he gathered all Judah and Benjamin, and the strangers with them out of Ephraim and Manasseh, and out of Simeon: for they fell to him out of Israel in abundance, when they saw that the LORD his God was with him.

He gathered all Judah and Benjamin. Not satisfied with these minor measures of purification and improvement, Asa meditated a grand scheme, which was, to pledge his whole kingdom to complete the work of reformation, and with this view waited for a general assembly of the people.

And the strangers with them out of Ephraim and Manasseh. The population of Asa's kingdom had been vastly increased by the continued influx of strangers, who, prompted by motives either of interest or of piety, sought in his dominions that security and freedom which they could not enjoy amid the complicated troubles which distracted Israel.

And out of Simeon. Although a portion of that tribe, located within the territory of Judah, were already subjects of the southern kingdom, the general body of the Simeonites had joined in forming the northern kingdom of Israel. But many of them now returned of their own accord.