He who spake in divers manners in old time, “hath in these last days spoken unto us by His Son.” This of course includes Christ’s personal teaching when on the earth, as we have it in the four Gospels of the New Testament. That was not, however, the whole of His teaching for the “last days,” for when He was about to leave the world, He said to His disciples, “I will pray the Father, and He shall give you another Comforter, that He may abide with you forever.” John 14:16. Respecting this Comforter, He said, “But the Comforter, which is the Holy Ghost, whom the Father will send in My name, He shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.” John 14:26. Again, “But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, He shall testify of Me.” John 15:26. SGL 9.3
Of the special work of the Comforter, which was to “abide forever,” the Saviour said: “It is expedient for you that I go away; for if go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you. And when He is come, He will reprove the world of sin, and of righteousness, and of judgment; of sin, because they believe not on Me; of righteousness, because I go to My Father, and ye see Me no more; of judgment, because the prince of this world is judged. I have yet many things to say unto you, but ye can not bear them now.” John 16:7-16. SGL 10.1
All that was to be spoken by the Son was not accomplished when He was here in person, for He said: “Howbeit when He, the Spirit of truth, is come, He will guide you into all truth; for He shall not speak of Himself; but whatsoever He shall hear, that shall He speak; and He will show you things to come. He shall glorify Me; for He shall receive of Mine, and shall show it unto you.” John 16:13-14. Of the coming and work of the Spirit, our Lord further spake to the disciples: “And, behold, I send the promise of My Father upon you; but tarry ye in the city of Jerusalem, until ye be endued with power from on high.” Luke 24:49. A record of the same conversation is also given in these words: “And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise (fulfilment of the promise) of the Father, which, saith He, ye heard of Me. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.” Acts 1:4-5. SGL 10.2