We do not love as we ought therefore our assurance must be on better grounds

"It is surely amazing that our opponents are unmoved by the many passages in the Scriptures that clearly attribute justification to faith and specifically deny it to works. Do they suppose that this is repeated so often for no reason? Do they suppose that these words fell from the Holy Spirit unawares? But they have thought up a piece of sophistry to evade them. They should be interpreted, so they say, as referring to 'faith fashioned by love,' that is, they do not attribute justification to faith except on account of love. Indeed, they do not attribute justification to faith at all, but only to love, because they imagine that faith can exist with mortal sin. Where does this end but with the abolition of the promise and a return to the law? If faith receives the forgiveness of sins on account of love, the forgiveness of sins will always be unsure, for we never love as much as we should. In fact, we do not love at all unless our hearts are sure that the forgivness of sins has been granted to us. If our opponents require us to trust in our own love for the forgiviness of sins and justification, they completely abolish the Gospel of the free forgiveness of sins. For men can neither render nor understand this love unless they believe that the forgiveness of sins is received freely." - The Book of Concords, Apology of the Augsburg Confession, Article 4. Justification, Pg. 122-123