It is the Spirit who gives life; the flesh is no help at all. The words that I have spoken to you are spirit and life. (John 6:63)
This means that the words were spoken by the Holy Spirit.
Christ does not want to give you the right to run to and fro in search of the Spirit, to lose yourself in reverie and say: “I have this by inspiration of the Holy Spirit.” Actually, it may be the devil who inspired you! Thus they alleged in the edict issued at the Diet of Augsburg [the “Confutation of the Augsburg Confession”]: “The church is holy; therefore it follows that its proclamations are holy and given by inspiration of the Holy Spirit.” Christ does not recognize such inspiration. He binds us solely to His Word. He does not want to see the Holy Spirit divorced from His Word. Whenever you hear anyone boast that he has something by inspiration of the Holy Spirit and it has no basis in God’s Word, no matter what it may be, tell him that this is the work of the devil. Christ does not bind you to anything but His mouth and His Word. He does not want to leave you wandering aimlessly about; He wants you to hear His Word. He declares: “The words which I speak are spiritual. Therefore if you want to obtain the Holy Spirit, you must adhere to My words; for they are spirit and life.”
Luther’s Works, vol. 23, 173
God has so ordered it that the Holy Spirit ordinarily comes through the Word. Christ Himself states this here. Therefore whenever you are faced by anything, even though it appears so attractive and holy that you imagine it to be an angelic being, take it, and hold it in the light of God’s Word. Examine it to see whether it is founded on Holy Writ, whether or not God has commanded and ordered it. If it is a mere fancy, a token of particular zeal, or a pious thought, but lacks God’s Word, spit on it. Unless it happens that God chooses to enlighten you particularly, as He did Moses, beware! Since God has confirmed the ministry of preaching, be on your guard against any such devotion or fancies as the devil may well suggest to you, even though they are sweet enough to induce you to weep large troughs full of tears.
… They were told that it was not permissible to resist the Holy Spirit. But I declare that I will resist Him if they do not have the Word of God. For John commands us (1 John 4:1) to test all the spirits to ascertain who is preaching and what is being preached.
But if I am to test a spirit, I must have the Word of God. The latter is to be the rule or touchstone or Lydian stone by which I can tell black from white and evil from good. It illumines everything, just as the sun does.
Luther’s Works, vol. 23, 174
Mark Christ’s words well: “It is the Spirit that gives life,” for these words leave us neither in doubt nor in error. Shortly after uttering these words He explains the meaning of the word “spirit.” He says: “The words that I have spoken to you are spirit.” This He does that we might beware of being misled by the treacherous preachers who come and boast about the Spirit, claiming to feel the compulsion of love and the spirit to preach. …
Perhaps you are tempted to ask: “Where does the Spirit give life? Or by what means? Where will I find the Spirit?” The reply is: “Hold to My words and speech. If you do that, you have the Spirit.” Thus the words are spirit in him who preaches and teaches, and also in him who hears and believes. A man is spirit in proportion to how much he adheres to the Word. On the other hand, he is flesh in proportion to his flesh and unbelief.
Luther’s Works, vol. 23, 175