In order to understand the remainder of this passage fully it is helpful to have some background in apostolic tradition, that is, the tradition by which the faith was handed down. It is significant that Jude uses these words - handed down - rather than indicating that Christians knew through revelation. The body of belief known as the faith was literally handed down from Christ, to the apostles, and then to Christians. The unfolding of this tradition is well recorded by John as he writes about the final Passover that Jesus shared with His disciples.
Much of chapters 13-17 of the book of John address this topic, but we will only look at a few significant passages. As Jesus gathered together with the twelve apostles He was aware that His hour had come, the end of His earthly ministry was near,
"These things I have spoken to you while abiding with you. But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you." (John 14:25-26)
It is important to remember to whom Jesus is speaking in this passage. He is speaking to the apostles, not to Christians of today. After all, has He spoken to me, while abiding with me in the twentieth or twenty-first century? Clearly He has not. The passage itself, as well as the context unmistakably indicates that He is speaking to the apostles. Additionally, note the universality of the revelation to the apostles. Jesus proclaims that all things will be taught and remembered with regard to His ministry and teaching. All that is known about God's plan of salvation and His will for us was entrusted to the apostles,
"When the Helper comes, whom I will send to you from the Father, that is the Spirit of truth who proceeds from the Father, He will testify about Me, and you will testify also, because you have been with Me from the beginning." (John 15:26-27)
If there is any question about to whom Jesus is speaking, or to whom the Spirit will be sent this should put it to rest. Again, the passage and the context make it beyond doubt that he spoke to the apostles only,
"But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. He will glorify Me, for He will take of Mine and will disclose it to you. All things that the Father has are Mine; therefore I said that He takes of Mine and will disclose it to you." (John 16:13-15)
The universality of revelation is restated here for the apostles. The truth will be plainly revealed to them by the Spirit. All things will be disclosed to them from the Father. Shortly after Jesus makes this statement He enters into a time of prayer as recorded in John, chapter 17. In this prayer He begins with a petition to glorify God through the glorification of the Son. Then in verse 6 He begins praying for the apostles, "the men whom You gave Me out of the world"...
"They are not of the world, even as I am not of the world. Sanctify them in the truth; Your word is truth. As You sent Me into the world, I also have sent them into the world. For their sakes I sanctify Myself, that they themselves also may be sanctified in truth. I do not ask on behalf of these alone, but for those also who believe in Me through their word;" (John 17:16-20)