During His mortal ministry, Jesus Christ taught His disciples the importance the Holy Ghost would portray in their lives. Because He, the greatest of all Comforters, remained with them, His disciples could not yet enjoy all the manifestations of the Holy Ghost for some reason yet to be revealed. To instruct them in the full measure of His doctrine, Jesus told them that there were many more things that He wanted to reveal to them, but could not because they were not ready to endure them. According to John, Jesus said toward that truth when He leaves them, "I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever... 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 14:16, 26
Though Jesus could not reveal all things to them at that time because they were not ready, He affirmed that the Holy Ghost would teach them all all things and help them to remember everything that He taught them. That promise applies to us in modern times.
Probably the last prophet with the apostolic mantle still in mortality at the time, Moroni affirms the teachings of the savior in his final testimony. Moroni, in testifying of his record to the world that became the Book of Mormon gave the key of the Spirit of God as the way to know the truth. He said nothing differently than did his Savior Jesus whom he saw as a resurrected being. Moroni's words are, "And by the power of the Holy Ghost ye may know the truth of all things." Moroni 10:5 What has Moroni seen of you?