Jamieson, Fausset & Brown

Behold therefore the goodness and severity of God: on them that fell, severity--in rejecting the chosen seed. but toward thee, goodness--"God's goodness" is the true reading, that is, His sovereign goodness in admitting thee to a covenant standing who before wert a "stranger to the covenants of promise" (Eph 2:12-20). if thou continue in his goodness--in believing dependence on that pure goodness which made thee what thou art.