“We rely not upon ourselves or our wisdom and ability. We preach not what we have ourselves invented. But this is our boast and trust in Christ before God, that we have made of you a divine epistle; have written upon your hearts, not our thoughts, but the Word of God. We are not, however, glorifying our own power, but the works and the power of Him Who has called and equipped us for such an office; from Whom proceeds all you have heard and believed.
It is a glory which every preacher may claim, to be able to say with full confidence of heart: ‘This trust have I toward God in Christ, that what I teach and preach is truly the Word of God.' Likewise, when he performs other official duties in the Church – baptizes a child, absolves and comforts a sinner, communes one with the body and blood of Christ – it must be done in the same firm conviction that such is the command of Christ.”
Martin Luther The Church Postils, vol. 8, pg. 226