In Revelation 3, the Holy Spirit's letter to Philadelphia (which is one of the two healthy churches, whom the Lord does not rebuke) says that the Lord has put a door in front of them that no one can shut (v8); He also promises to keep them from the hour of testing that is about to come upon the whole earth (v10). You’d think they’re a lock to win the prize, having the Lord hold the door open, as it were, and a promise to be kept from the hour of testing. What is the instruction to believers who have a door opened by the Lord Himself, and are promised sovereign protection from the hour of testing? Is it “sit back and relax”? No! It’s “hold fast what you have”!
I am coming quickly; hold fast what you have, so that no one will take your crown.”
‭‭Revelation‬ ‭3:11‬ ‭NASB‬‬
They are pleasing to the Lord, and He's made precious promises concerning their future, and yet the Lord says they’re still in danger: they can still lose their crown! And so they are commanded to hold fast what they have received. That’s what our gatherings as a church ought to stir up: a longing to hold fast, a longing to kindle afresh the gift we’ve been given. Those who are fiery hot, hold fast. Just as we know that God would rather us be cold than lukewarm (Revelation 3:16), that it is a sin to not be on fire for the Lord, I want to say, it is a sin to not hold fast what we have! And more than that, there's a danger: “For to everyone who (holds fast what he) has, more shall be given, and he will have an abundance; but from the one who does not (hold fast what he has), even what he does have shall be taken away” (Matthew‬ ‭25:29‬ ‭- paraphrase).

Holding Fast Is How We Bear Fruit

A precious brother seeking a godly life recently asked: "How does the new covenant life bear fruit? When will we see all that has been gloriously proclaimed to us for ourselves?" I was reminded of the parable of the four soils, and how Jesus explained what was different about the only ultimately fruitful soil: 
“But the seed in the good soil, these are the ones who have heard the word in an honest and good heart, and hold it fast, and bear fruit with perseverance.”
‭‭Luke‬ ‭8:15‬ ‭NASB‬‬
He says the good soil bears fruit by persevering. Hopefully that begs the question: persevering in what? Persevering in holding fast to the word it received! The fruit of the new covenant only comes as we persevere in holding fast to the word that we have heard. 
My experience is that the enemy absolutely hates the truth of the new covenant. He will send trials and afflictions and doubts and difficulties all with the sole purpose of ripping the seed from the soil of our hearts. We have to learn to hold fast to the word we have received in the face of these forces that try to rip it from our grasp!
Hostile forces will try to rip the truth from our grasp, but praise be to God, it is the exercise of our faith — tenaciously holding to and proclaiming what we have heard — that produces the fruit. In God’s glorious design, the enemy’s opposition accomplishes the exact opposite of his intended impact! His opposition actually enables us to fulfill the command to hold fast, and thus bear fruit. 
We must not grow weary of holding fast. We will reap a harvest if we don’t give up (Galatians 6:9). 
How do I know? That is what God’s Word declares! And further, it is the testimony of the godly men that I know. They’re reaping a harvest, and they’re sowing seeds of hope from that same harvest into our lives and church as well. 

We have a responsibility to those who sow into our lives

Watch yourselves, that you do not lose what we have accomplished, but that you may receive a full reward.”
‭‭2 John‬ ‭1:8‬ ‭NASB‬‬
This takes the Holy Spirit’s instruction to the Philippians one step higher. There, Paul says, “let us keep living by that same standard to which we have attained” ‭‭(Philippians‬ ‭3:16‬)‬‬; but here, John says, essentially, not “let us keep living by the standard we have attained,” but “you also keep living to the standard that we have attained.” We have a solemn responsibility to safeguard that which has been entrusted to us by other faithful men. And it’s not optional, either. The stakes are the same as we saw earlier: “that you may receive a full reward...” (just like, “so that no one will take your crown” in Revelation 3:11). 
One man may attain to a godly life in the new covenant, but another can effectively lose the ground that was gained by failing to hold fast the standard that was proclaimed. The teaching we have received is a precious legacy and inheritance, and I want to be diligent and take pains to hold it fast, not only so that I might receive the reward (Rev 3:11), and not only that I might bear fruit (Luke 8:15), but also that the ground that others have gained in this new covenant age might not be lost. It is a solemn responsibility, and we can be confident that as we take His command seriously, He will do abundantly beyond all that we ask or think, and continue to lead His church from one degree of glory to the next. The thing that encourages me is that God’s Word says we can enjoy this life now, as we hold fast. “For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end” (Hebrews‬ ‭3:14‬ ‭NASB‬‬).
As we hold fast to the word we have received, we can presently partake of the life of Jesus Christ. We don’t have to wait: the partaking accompanies holding fast. Praise God.