"Now I desire to remind you, though you know all things once for all, that the Lord, after saving a people out of the land of Egypt, subsequently destroyed those who did not believe. And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day, just as Sodom and Gomorrah and the cities around them, since they in the same way as these indulged in gross immorality and went after strange flesh, are exhibited as an example in undergoing the punishment of eternal fire." (Jude 5-7)Its hard to fully understand exactly what Jude ment by his desire to remind them what they themselves either once knew or knew once for all times. Even the translators are mixes as to exactly what Jude was driving at. He could have been simply reminding them of the truth they already new; of the truth they had received and that had been illuminated to them by the Holy Spirit. For as John said, "As for you, the anointing which you received from Him abides in you, and you have no need for anyone to teach you; but as His anointing teaches you about all things, and is true and is not a lie, and just as it has taught you, you abide in Him." (1 John 2:27) Even though the Holy Spirit teaches us and instructs our heart, we all need times to be reminded of what we have learned and of the things we already know. Peter also understood our constant need to be reminded. "Therefore, I will always be ready to remind you of these things, even though you already know them, and have been established in the truth which is present with you. I consider it right, as long as I am in this earthly dwelling, to stir you up by way of reminder,.. And I will also be diligent that at any time after my departure you will be able to call these things to mind." (2 Peter 1:12-15) It is through this constant reminding that a truth becomes so fixed in our mind that it is ever ready to spring forth in full power and persuasion to benefit us in our moment of need.
It is also possible that Jude was reminding them, not just to return the knowledge to the forefront of their mind, but to remind them that they might make a new connection to the truth. Sometimes we can know the truth yet fail to make the connection of it to our lives. Sometimes we need to be reminded of the truth that we may reconsider it afresh in our minds that we might discover some new way to apply it to our everyday lives and to the circumstances we face. For example, I'm sure many believers reading Jude's letter already knew the story of the Israelites leaving Egypt and how a whole generation died in the wilderness on their way to the promise land, but had they ever stopped to consider what such truth meant for their lives then? Could they too become like those who left Egypt but failed to make it to the promised land?
Jude is reminding them that not all who start out well, finish well. All the Israelites left Egypt, but very few of that generation actually stepped foot into the promise land. The angles have great position with God, standing before His presence, but even some of them were rejected when defects were found in their nature. Its not how you start out that is important, but how you finish.
The truth is, "many are called, but few are chosen." (Matthew 22:14) So how does one progress from being called to being chosen? Jesus spoke these words at the end of a parable of a king who had prepared a great wedding feast for his son. The call had gone out to all that they might come and feast with the king and his son. "But when the king came in to look over the dinner guests, he saw a man there who was not dressed in wedding clothes, and he said to him, 'Friend, how did you come in here without wedding clothes?' And the man was speechless. Then the king said to the servants, 'Bind him hand and foot, and throw him into the outer darkness; in that place there will be weeping and gnashing of teeth.'" (Matthew 22:11-13) John, in his Apocalypse, reveals a bride who had made herself ready for the King's Son, "Let us rejoice and be glad and give the glory to Him, for the marriage of the Lamb has come and His bride has made herself ready. It was given to her to clothe herself in fine linen, bright and clean; for the fine linen is the righteous acts of the saints." (Revelation 19:7-8) How does one prepare himself for the wedding feast to come? How does one move from being called to being chosen? By putting on righteousness. By daily showing forth the acts of righteousness that flow from our converted soul and our spirit that has been born again. How does one make himself ready? By living the life he was meant to live; a life born anew in Jesus Christ.