How do you become one with Christ?

Luther:

By faith. Faith contracts, as it were, a marriage between Christ as the bridegroom and the soul as the bride. That is such a perfect marriage that human marriage is only a weak reflection of it. It is a marriage in the community of goods, both the good as well as the bad. The soul is full of sins, death and conviction. Christ is full of grace, life and salvation. When faith comes between both, then everything from the bride comes also from the bridegroom and everything from the bridegroom also from the bride.

In this marriage there is not only communion, but furthermore also conflict and conquest. Because Christ is also God. His righteousness, life and salvation are invincible, eternal and almighty. Therefore sin, hell and death can never overcome Him. When He takes the bride as his wife through the wedding ring of faith, then the soul thereby becomes free of all sins, safeguarded for death and filled with eternal righteousness.

Christ gains thereby a gorgeous bride without spot or wrinkle for Himself. The rich and godfearing bridegroom takes a poor and godless whore as his wife and beautifies her with all his blessings.