Thought from Luther on John 6:27 "For on Him has God the Father set His seal"
"This seal is Christ and no other; with these words God rejects and condemns all other seals. These words are significant and comprehensive in scope, for they assert that he who wants to be saved must possess this food which the Son supplies; he must be found in the Son, on whom God has set His seal. Without Him he will forfeit eternal life. For here we find the seal and the testimony."

"God does not have His Gospel proclaimed for the sake of the belly, but for the welfare and the salvation of our souls."

"We must hold to this chief doctrine. No matter how the verses dealing with good works are manipulated, as learned men are able to do in order to mystify poor, naive people, this text cannot be ignored and disavowed."

Taken from Luther's Works vol. 23

These quotes from Luther concerning John 6 determines everything that the church believes, teaches, and confesses. Christ Himself is the seal of the Father. He is the seal by which death is conquered, the ransom for sin paid, and the wrath of the Father satisfied. Christ Himself is the only means by which fallen man has forgiveness of sin, life, and salvation. Outside of Christ Jesus there is no salvation.
This determines the proclamation of the Gospel of Jesus Christ. The preacher is not to speak about Jesus, or use Jesus as a great example of how to live a good moral life. The sermon is not to end with an exhortation to good works, nor is it to end with a hopefulness of salvation. The preacher takes God's Holy Word and seals the foreheads of all gathered. He is the great angel that seals the foreheads of God's elect before the Final Judgment. (Revelation 7:2-3). The preacher is placed in the pulpit to do nothing more or less than to seal the believers with the seal that is Christ the Lord. He is to baptize, commune, and preach the seal that is Christ Jesus. What a great polemic against the religion of the law and the devil this text is. Christ Jesus is the seal of the Father. He is the End of the Law and the seal of salvation for fallen man. Let us desire to preach nothing but Christ crucified, for in the death of Christ Jesus, we have the seal of the Father. God is faithful. We know this only through the blood of Christ Jesus, the seal of God the Father, Amen.