Preach, therefore, by word. Preach also by action. Preach the Gospel at home, and preach it every where. But always remember, in connection with a deep sense of human infirmity and liability to error, to preach it under the guidance, and by the power, of the Holy Spirit. The message is from God, and God only. To be united with God in proclaiming his messages, we must speak as God gives us utterance. It is important, in this age of the world, when we hope the millennial period is rapidly approaching, to revive and to act upon the great primitive truth, that holy men speak as they are moved by the Holy Ghost; and that, so far as they are holy, they have no power of speaking otherwise.
So far as we are in union with God, while it is true that we ourselves may be said to speak, it is equally true that God speaks in us. There is but one true voice. The voice which speaks at the centre, if it embodies the truth, is the same voice which speaks at the circumference, and which speaks everywhere else. It is incapable of change. It speaks in the star, the flower, the falling leaf, the ocean's wave, in the winds, in the thunder, in the sound of the falling water, in the true philosopher, in the true poet, in the true preacher, in the Bible, everywhere the same in import, though various and differing in manifestation. When, therefore, we are in God by a true unity of spirit, we speak as God would have us speak, and by the inspiration of the Almighty.
And truly holy men, in all ages of the world, have known, by their inward experience, the truth of what has now been said; and they have not hesitated to proclaim what they have known. What was the language of the ancient prophets? What was the language of Paul? Everywhere does he discard the idea that his teaching is from himself. Everywhere does he discard all confidence in human wisdom. Prophets and apostles, by their own acknowledgment, were only instruments, which gave form and locality to the divine utterance. Holy men, in all subsequent ages, have felt and spoken in the same way. The records of the interior or experimental history of the church show this to be the case. In all periods of great religious attention, and in all cases of deep religious experience, language is used by those who are the subjects of such experience, which corresponds to the fact of the divine origination of all that is true and right in the soul. The human in men may be said at such times to be kept, as it is sometimes expressed, in abeyance; or, what is better, to be placed under a divine and holy direction. While they are conscious of personal responsibility, it is still true that they utter what is given them. It is worthy of notice, that language, which, in religion as well as in philosophy, is an index of the mind's operations, often takes at such times the passive instead of the active form; — implying, while it does not exclude the idea of activity, especially of cooperative action, that we are also the subjects of action.
— edited from A Treatise on Divine Union (1851) Part 7, Chapter 4 by Thomas Cogswell Upham. His blog is managed by Craig L Adams and can be found here: http://thomascupham.blogspot.com