Paul has just told us that the love of God has been poured out within our hearts through the Holy Spirit who was given to us. That is subjective. You might say experiential. It’s something the believer comes to know internally—and it is an assurance both of God’s presence, ownership, and that we are loved. But now Paul anchors that subjective assurance in something more objective. He doesn’t leave the love of God in the realm of emotion or resting on our fluctuating feelings. He grounds it in objective redemptive history–in an undeniable event. He shows us what that love did. The cross is not merely a symbol of divine love, it is the objective, public demonstration of it. It is God stepping into time and proving what kind of love He has for mankind.