True Christianity, Book 1, Chapter 5, "What True Faith Is" by Johann Arndt

He who believes that Jesus is the Christ is a child of God (1 John 5:1).

            Faith is a deep assent and unhesitating trust in God's grace promised in Christ and in the forgiveness of sins and life eternal. It is ignited by the Word of God and the Holy Spirit. Through this faith we receive forgiveness of sins, in no other way than through pure grace without any of our own merits (For by grace you are saved through faith, and this is not from yourselves, it is the gift of God; Ephesians 2:8) but only by the merits of Christ. For this reason, our faith has a certain ground which is true, continual, and eternal before God. It is not the righteousness of an angel but of the obedience, merit, and blood of Christ and becomes ours through faith. Even if it is weak and we are still hemmed around by many sins, these are covered over out of grace for Christ's sake.

            By this deep trust and heartfelt assent, man gives his heart completely and utterly to God, rests in God alone, gives himself over to God, clings to God alone, unites himself with God, is a participant of all that which is God and Christ, becomes one spirit with God, receives from him new power, new life, new consolation, peace and joy, rest of soul, righteousness and holiness, and also, from God through faith, man is newborn. Where new faith is, there is Christ with all his righteousness, holiness, redemption, merit, grace, forgiveness of sins, childhood of God, inheritance of eternal life. This is the new birth that comes from faith in Christ. Therefore, the Epistle to the Hebrews, 11.1 calls faith a substance or a certain assurance of things on which man hopes and a conviction of things man does not see. The consolation of living faith becomes powerful in the heart; it convinces the heart that one find's in one's soul heavenly goodness, namely, rest and   peace in God, so certain and true that one might then die with a happy heart. This is the strength in the spirit, in the internal man and the joyousness of faith, or parrhisia [confidence]( Eph 3:12; Phil 1:20; 1 Jn 2:28; 3:21) [1], that is, joyousness in God (1 Thessalonians 2:2) and plerophoria, a completely unhesitant certainty (1 Thess 1:5).[2]

            When I am to die this faith must strengthen me in my soul and must assure me internally by the Holy Spirit. It must be an inner, living, eternal consolation; it must hold me and strengthen me also as a supernatural, divine, heavenly power to conquer death and the world in me, and there must be such an assurance and union with Christ that is able to stand in either death or life (Because of this, in fact, I suffer as I do. But I am not ashamed, because I know the one in whom my faith is set and am convinced that he is able to protect what has been entrusted to me until that day.  Timothy 1:12; And we know that all things work together for good for those who love God, who are called according to his purpose. Romans 8:28) Therefore, John [in 1 John 5:4] says: "Everything which is born of God conquers the world."

            Everything that is born of God is truly no shadowy work, but a true life work. God will not bring forth a dead fruit, a lifeless and powerless work, but a living, new man must be born from the living God. Our faith is the victory that conquers the world.

            That which man is to conquer must be a mighty power. If faith is to be victorious over the world, it must be a living, victorious, active, working, divine power; indeed Christ must do everything through faith.

            Through this power of God we are once more drawn into God, inclined toward God, transplanted and set in God, taken out of Adam and as a cursed vine placed in Christ the blessed and living vine (Jn 15:4). Thus, in Christ we possess all his goods and are made righteous in him.

            Just as a graft is set in a good stem and grows, blossoms, and brings forth fruit in it, but out of it dies, so a man outside of Christ is nothing but a cursed vine and all his works are sins (Deut 32:32-33); Their grapes are grapes of poison. In Christ, however, he is righteous and holy. Therefore, Paul in 2 Corinthians 5:21 says: "For or sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God."

            From this you now see that works cannot make you righteous. First, you must be established in Christ through faith and be righteous in him before yo can do any good work. See to it indeed that your righteousness is the grace and gift of God that comes before all your merit. How can a dead man walk, stand, or do anything good unless someone does not first make him living? Thus, since you are dead in sins and dead before God you can do no work pleasing to God unless you are first made living in Christ.

            Righteousness comes alone from Christ through faith, for faith is in a man as a newborn, small, naked, and simple child that stands unclothed simply before his Redeemer and Sanctifier, and receives all from him who begot it, namely righteousness, piety, holiness, grace, and the Holy Spirit.

            Thus, if this naked, simple child is to be clothed with God's mercy, it must both lift  its hands up and receive everything from God, grace together with all holiness and piety. Receiving this, it is made pious, holy, and blessed.

            Therefore, righteousness comes only by faith and not from works. Indeed, faith receives Christ and makes him his own with all those things which he is and has. You must turn from sin, death, the devil and hell. If you have all the sin of the world upon yourself, it cannot harm you, for so strong, mighty, and living is Christ in you with his merits through faith.

            Since Christ now lives and dwells in you through faith, his indwelling is not a dead work but a living work. As a result, the renewal from Christ through faith comes about. Grace brings about two things in you: first, faith places Christ in you and make you his possession; second, it renews you in Christ so that you grow, blossom and live in him. What is the use of a graft in a stem if it does not grow and bear forth fruit? Just as once through Adam's fall, through the deception and treachery of the devil, the seed of the serpent was sowed in man - that is, the evil, satanic pattern of life out of which an evil, poisonous fruit grew - so by God's word and the holy Spirit faith was sowed in man as a seed of God in which all divine virtues, qualities and characteristics, in  a hidden manner, were contained and grew out to a beautiful and new image of God, to a beautiful new tree on which are the fruits of love, patience, humility, meekness, patience, peace, chastity, righteousness, the new man and the whole kingdom of God. The true sanctifying faith renews the whole man, purifies the heart, unites with God, makes the heart free from earthly things, hungers and thirst after righteousness, works love, gives peace, joy, patience, consolation in all suffering, conquers the world, makes children heirs of God and of all heavenly eternal goods and co-heirs of Christ. If you find someone who does not have the joy of faith but is weak of faith and seeks comfort, do not reject him because of this but comfort him in the promised grace of Christ. This always remains firm, certain, and eternal. If we fall in weakness and stumble, God's grace does not fall away if we rise again through true repentance. Christ remains always Christ and the Sanctifier. He may be grasped with weak or with strong faith. Weak faith belongs as much to Christ as strong. Whether a man is weak or strong of faith, he is Christ's own just the same. The grace that is promised is common to all Christians and is eternal. On this, faith must rest, whether it be weak or strong. In his time, God will allow you to come to refreshing, joyous consolation, whether he bring it to your heart in a short time or in a longer period (Ps 32:2-5, 77:8-11). On this see Book II.