'Matter' in the Bible

because they bring trouble to you by their treachery with which they have deceived you in the matter of Peor, and in the matter of Cozbi, the daughter of a prince of Midian, their sister, who was killed on the day of the plague that happened as a result of Peor."

Look, these people through the counsel of Balaam caused the Israelites to act treacherously against the Lord in the matter of Peor -- which resulted in the plague among the community of the Lord!