“Faith does not merely mean that the soul realizes that the divine word is full of all grace, free and holy; it also unites the soul with Christ, as a bride is united with your bridegroom. From such a marriage, as St. Paul says (Eph 5:31-32) it follows that Christ and the soul become one body, so that they hold all things in common, whether for better or worse.
This means that what Christ possesses belongs to the believing soul; and what the soul possesses, belongs to Christ. Thus Christ possesses all good things and holiness; these now belong to the soul. The soul possesses lots of vices and sin; these now belong to Christ. Here we have a happy exchange and struggle.
Christ is God and human being, who has never sinned and whose holiness is unconquerable, eternal, and almighty. So he makes the sin of the believing soul his own through its wedding ring, which is faith, and acts as if he had done it [i.e. sin] himself, so that sin could be swallowed up in him. For his unconquerable righteousness is too strong for all sin, so that is made single and free from all its sins on account of its pledge, that is its faith, and can turn to the eternal righteousness of the bridegroom, Christ. Now is not this a happy business?
Christ, the rich, noble, and holy bridegroom, takes in marriage this poor, contemptible, and sinful little prostitute, takes away all her evil, and bestows all his goodness upon her! It is no longer possible for sin to overwhelm her, for she is now found in Christ and his swallowed up by him, so that she possesses a rich righteousness and her bridegroom.”
(Martin Luther, The Liberty of a Christian; in D. Martin Luthers Werke: Kritische Gesamtausgabe, vol. 7, 25.26-26.9 – cited in The Christian Theology Reader, edited by Alister McGrath, 3rd Edition, 2008, 441-442)
“Here is a trustworthy saying that deserves full acceptance: Christ Jesus came into the world to save sinners—of whom I am the worst.”
-1 Timothy 1:15