The old Adam counts God as his enemy...

Likewise, we believe, teach, and confess that the unregenerate will of mankind is not only turned away from God, but also has become God's enemy.  So it only has an inclination and desire for that which is evil and contrary to God, as it is written in Genesis 8:21, "The intention of man's heart is evil from his youth."
~BOC, FE, II, 3

The Lord's Supper is the food of souls...

On this account it is indeed called a food of souls, which nourishes and strengthens the new man.  For by Baptism we are first born anew.  But, as we have said before, there still remains the old vicious nature of flesh and blood in mankind.  There are so many hindrances and temptations of the devil and of the world that we often become weary and faint, and sometimes we also stumble.

Therefore, the Sacrament is given as a daily pasture and sustenance, that faith may refresh and strengthen itself so that it will not fall back in such a battle, but become ever stronger and stronger. The new life must be guided so that it continually increases and progresses.  But it must suffer much opposition.  For the devil is such a furious enemy.  When he sees that we oppose him and attack the old man, and that he cannot topple us over by force, he prowls and moves about on all sides.  He tries every trick and does not stop until he finally wears us out, so that we either renounce our faith or throw up our hands and put up our feet, becoming indifferent or impatient.  Now to this purpose the comfort of the Sacrament is given when the heart feels that the burden is becoming too heavy, so that it may gain here new power and refreshment.
~BOC, LC, V, 24-27

A truly Christian life is nothing other than a daily Baptism...

Lastly, we must also know what Baptism signifies and why God has ordained just this outward sign and ceremony for the Sacrament by which we are first received into the Christian Church.  The act or ceremony is this: we are sunk under the water, which passes over us, and afterward are drawn out again.  These two parts, (a) to be sunk under the water and (b) drawn out again, signify Baptism's power and work.  It is nothing other than putting to death the old Adam and affecting the new man's resurrection after that.  Both of these things must take place in us all our lives.  So, a truly Christian life is nothing other than a daily Baptism, once begun and ever to be continued.  For this must be done without ceasing, that we always keep purging away whatever belongs to the old Adam.  Then what belongs to the new man may come forth.

~BOC, LC, IV, 64-65

The work of the old Adam...

In short, the old Adam encourages us to have all kinds of evil lusts, which cling to us by nature and to which we are moved by the society, the example, and what we hear and see of other people. They often wound and inflame even an innocent heart.
~BOC, LC, III, 102