"Now, the Lord in this passage speaks, in particular, of preachers or prophets, whose real and proper fruit is nothing else that this, that they diligently proclaim this will of God to the people and teach them that God is gracious and merciful and has no pleasure in the death of a sinner, but wants him to live, moreover, that God has manifested His mercy by having His only-begotten Son become man. Whoever, now, receives Him and believes in Him, that is, whoever takes comfort in the fact for the sake of His Son, God will be merciful to him, will forgive his sins, and grant him eternal salvation, etc.,--whoever is engaged in this preaching of the pure Gospel and thus directs men to Christ, the only Mediator between God and men, he, as a preacher, is doing the will of God. That is the genuine fruit by which no is deceived or duped. For if it were possible that the devil were to preach this truth, the preaching would not be false or made up of lies and a person believing it would have what it promises.--After this fruit, which is the principal and most reliable one and cannot deceive, there follow in the course of time other fruits, namely, a life in beautiful harmony with this doctrine and in no way contrary to it. But these fruits are to be regarded as genuine fruits only where the first fruit, namely, the doctrine of Christ, already exists." Martin Luther, St. L., XIII, pp. 800ff.
C. F. W. Walther, The Proper Distinction between Law and Gospel, trans., W. H. T. Dau, St. Louis: Concordia Publishing House, 1928, p. 413. Matthew 7:21.