The clause, “So guard yourselves in your spirit…,” makes it evident that the Jewish men were endangering their own spiritual well being as a result of their marriages to pagan women. These pagan wives could and likely would turn these men away from following the Lord to serve their gods (Joshua 23:12; Deuteronomy 7:4). Therefore the prophet Malachi issues a stern warning to the men, to take heed to their spirits (or to guard their hearts) lest they be led astray. He warns them not to be faithless to the wives whom they married in their younger years.
15 Did he not make them one, with a portion of the Spirit in their union? And what was the one God seeking? Godly offspring. So guard yourselves in your spirit, and let none of you be faithless to the wife of your youth.