I decided to know nothing among you except Jesus Christ and Him crucified.
1 CORINTHIANS 2:2
God's Ordered Power
You should direct your attention to the ordered power of God and the ministrations of God; for we do not want to deal with the uncovered God, whose ways are inscrutable and whose judgments are unsearchable (Romans 11:33). We must reflect on God's ordered power, that is, on the incarnate Son, in whom are hidden all the treasures of the Godhead (Colossians 2:3). Let us go to the child lying in the lap of His mother Mary and to the sacrificial victim suspended on the cross; there we shall really behold God, and there we shall look into His very heart. We shall see that He is compassionate and does not desire the death of the sinner, but that the sinner should "turn from his way and live" (Ezekiel 33:11). From such speculation or contemplation spring true peace and true joy of heart. Therefore Paul says (1 Corinthians 2:2): "I determine to know nothing except Christ." We have leisure to speculate on this with profit.
From Lectures on Genesis (Luther's Works 3:276-77)
The desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh.
The Battle of the Flesh
When someone becomes aware of this battle of the flesh, he should not lose heart on this account; but by the Spirit he should fight back and say: "I am a sinner, and I am aware of my sin; for I have not yet put off my flesh, to which sin will cling as long as it lives. But I will obey the Spirit rather than the flesh. That is, by faith and hope I will take hold of Christ. I will fortify myself with His Word, and thus fortified I will refuse to gratify the desires of the flesh." . . . Thus there is great comfort for the faithful in this teaching of Paul's, because they know that they have partly flesh and partly Spirit, but in such a way that the Spirit rules and the flesh is subordinate, that righteousness is supreme and sin is a servant.
From Lectures on Galatians (1535) (Luther's Works 27:72-74)
Therefore be imitators of God, as beloved children. And walk in love, as Christ loved us and gave Himself up for us, a fragrant offering and sacrifice to God.
EPHESIAN S 5:1-2
This fruit is that, just as we have eaten and drunk the Lord Christ's body and blood, we in turn let ourselves be eaten and drunk, and also speak to our neighbor the words: "Take, eat and drink." This is not a mockery, but entirely serious: that you give yourself with all your life, just as Christ has done for you in these words, with all that He is. It is as if He would say: "I Myself am here given for you. I present you with this treasure. What I have, you also shall have. When you are in want, I also will be in want. Here you have My righteousness, life, and salvation, so that neither sin nor death, neither hell nor any distress will overcome you. As long as I am righteous and alive, you also will remain godly and alive." He speaks these words to us. We must also lay hold of them and speak them to our neighbors, not only with our mouths but also with our deeds; in this way: "Look, my dear brother, I have received my Lord. He is mine, and I now have enough of all fullness and to spare. So take what I have, and everything will be yours; I put it at your disposal. If it is necessary that I die for you, I will also do it." This goal is placed before us in this Sacrament, so that this proof [of love] toward our neighbor may appear in us.
From Sermon on Confession and the Sacrament (Luther's Works 76:444-45)
Be gracious to me, O LORD, for I am languishing; heal me, O LORD, for my bones are troubled. My soul also is greatly troubled.
The Anxious Heart
We also see many godly hearts that are always sad and dejected, being anxious and troubled at their own thoughts, despairing in their temptations from the devil. "Where," say the world and our own flesh, "is the Holy Spirit about whom you Christians boast?" Therefore, a Christian should be wise here so that he does not pass sentence and judge according to his own thoughts and feelings, but know that against just this temptation and weakness he must cling to the Word and the comforting sermon which the Holy Spirit preaches to all poor, distressed hearts and consciences. Christ says about the office which He is to carry out through the Holy Spirit: "The Spirit of the Lord is with Me. Because the Lord has anointed Me, He has sent Me to preach to the wretched, to bind up the broken hearts, likewise to comfort all the sorrowful" (Isaiah 61:1-2). From this you should learn (as you hear at this place and everywhere in the Gospel) that God does not want to have you sad and frightened, but cheerful and confident of the sure, truthful promise of His grace, which the Holy Spirit Himself preaches to you.
From the Church Postil, sermon for Pentecost on John 14:23-31 (Luther's Works 77:334-35)