Gerhard and Walther

It is by faith alone that we are made partakers of this blessed spiritual union, as it is written, “I will betroth thee unto me in faithfulness.” By faith we are engrafted as branches into Christ, the spiritual Vine, so that we derive all our life and strength from Him; and as those united in marriage are longer twain, but one flesh, so “he that is joined to the Lord is one spirit,” because Christ dwells in our hearts by faith.—Johann Gerhard, Sacred Meditation XIII

In justification, sin is forgiven, but it retains its roots in our heart. Therefore, if a Christian does not daily renew himself, his heart will again become wild, like tree that is not pruned or a garden in which weeds are not removed.—C. F. W. Walther, God Grant It!, p. 796.