From the Word
15 “Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. 16 You will know them by their fruits. Are grapes gathered from thorns, or figs from thistles? 17 So, every sound tree bears good fruit, but the bad tree bears evil fruit. 18 A sound tree cannot bear evil fruit, nor can a bad tree bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Thus you will know them by their fruits.
Matthew 7:15–20, RSV
No one knows others by their fruits, except he who is born again. Therefore he who has not the Spirit cannot have this knowledge. The fruit by which we are to know them is unbelief. One can know them to some extent by open sins, yet this judgment is deceptive, for Christians also fall. Hence the fruit by which they are known is an inner fruit, and I must have the Holy Spirit and judge according to his guidance. The fleshly eye and reason are not sufficient. You may see two persons go to the Lord’s Supper, the one is a believer, the other not, and yet their external work is the same. What then makes the difference? Faith in the heart and unbelief, because the one regards it as a good work, the other not. Tauler said that believers and unbelievers were often so similar in external appearance that no one could distinguish them, nor is reason able to judge unless we have the Spirit of God. For this reason the decision and judgment of spiritual things should not be based on external things, as on the work and person, but on the inner condition of the heart. The fruits and the good works do not make any one pious or good; but he must first be good and pious at heart. The apples do not make the tree, but the tree must be there before the fruit.
If I understand this, then I notice that there is no work so bad as necessarily to condemn a man, nor none so good as to save him. Faith alone saves us, and unbelief alone condemns us. The deed of the adulterer does not condemn him; the adultery only shows that he has fallen from the faith, and this condemns him. Nothing makes any one good but faith, and nothing makes one wicked but unbelief. The tree shall be cut down; he does not say, the fruit shall be cut down. Thus the works of love do not make me good, but faith, in which I do these works and bear this fruit. We must begin with faith, but the pope begins with works, just as though I could bear apples before I was an apple tree. Thus unbelief is the foundation of all sin.
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, pp. 306–07.