Who is it that overcomes the world except the one who believes that Jesus is the Son of God?
The real, victorious faith is the one that believes that Jesus Christ is God's Son. It is an invincible power worked in the hearts of Christians through the Holy Spirit. It understands with certainty and does not flutter back and forth or gape at its own thoughts, but takes hold of God in Christ as His Son sent from heaven, through whom He reveals His will and heart and delivers us from sin and death to grace and new, eternal life. This confidence and trust, which does not rely on its own merit or worthiness but on Christ, the Son of God, and on His might and power, contends against the world and the devil. Therefore, this faith is not a cold, worthless, empty, and idle thought . . . but a living, active power, so that wherever it is, its fruit, victory, and conquest must follow—or, if they do not follow, then faith and the new birth are not there.
From the Church Postil, sermon for the Sunday after Easter on 1 John 5:4-12 (Luther's Works 77:119-20)
"So the last will be first, and the first last."
Our Hope Is in Him
So here is the summary of this Gospel: no one is so high, or will get so high, that he does not have to be afraid that he may become the very lowest (see 1 Corinthians 10:12). On the other hand, no one has fallen so deeply, or can fall so deeply, that he cannot hope to become the highest, because all merit is abolished and only God's goodness is praised. It has been determined most surely: "The first shall be the last, and the last shall be the first." When He says, "The first shall be the last," He takes away all your arrogance and forbids you to exalt yourself above any prostitute, even if you were Abraham, David, Peter, or Paul. But when He says, "The last shall be the first," He prevents all your despair and forbids you to cast yourself below any saint, even if you were Pilate, Herod, Sodom, and Gomorrah.
From the Church Postil, sermon for Septuagesima on Matthew 20:1-16 (Luther's Works 76:319)
If anyone does sin, we have an advocate with the Father, Jesus Christ the righteous.
1 JOHN 2:1
The Devil's Nature
And this is the devil's nature and true color by which he should be recognized. He is such a wicked spirit that he can make great sins so small that they will not be seen; and, on the other hand, he can make small sins great, so that someone gnaws at, torments, and kills himself over them. Therefore, a Christian ought to learn not to let his conscience be easily troubled. But if he believes in Christ and desires to be righteous, if he contends against sin as much as he can, yet in the expectation that at times he will trip and stumble, then he does not allow such stumbling to destroy his good conscience. Instead, he says, “May this wrongdoing and stumbling depart from me along with the other infirmities and sins that I must include in the article of the Creed, 'I believe in the forgiveness of sins,' and the Fifth Petition of the Our Father, 'Forgive us our trespasses.' "
From Sermons on the Gospel of John, Chapters 17-20 (Luther's Works 69:203)
I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord.
Why God Gives Us Faith
God gives faith not so that it can deal with insignificant things, but [so that it can deal with] things the whole world cannot handle, such as death, sin, world, and devil. All the world cannot stand against death, but flees from it and is terrified of it and also is overcome by it; but faith stands firm, opposes death (which consumes all the world), prevails over it, and even swallows that which insatiably consumes life (see 1 Corinthians 15:54). So also all the world cannot constrain or suppress the flesh; rather, it rules over all the world, and what it wants must happen, so that all the world becomes fleshly through it. But faith attacks it, subjugates it, and bridles it, so that it must serve. Likewise, no one can endure the world's raging, persecuting, slandering, desecrating, hatred, and jealousy. Everyone yields and becomes weak, while [the world] conquers and wins. Except faith mocks them and tramples them with its feet and turns them into joy and delight.
From the Church Postil, sermon for Epiphany 4 on Matthew 8:23-27 (Luther's Works 76:284)