In this case, it is Christ’s promise that the visible Church he established will always teach the Truth.
“Careful to keep the unity of the Spirit in the bond of peace. One body and one Spirit: as you are called in one hope of your calling. One Lord, one faith, one baptism.” Eph 4:3-5
The Church is to be one, not many.
“And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it.” Mat 6:18
Here Christ says his Church will not be destroyed.
“I have yet many things to say unto you, but ye cannot bear them now. 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 shew you things to come.” John 16:12-13
“But if I tarry long, that thou mayest know how thou oughtest to behave thyself in the house of God, which is the church of the living God, the pillar and foundation of the truth.” 1Tim 3:15
Here Christ says, and Paul reaffirms, that the Church will always teach the TRUTH.
Then Jesus came to them and said, "All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."Mat 28:18-20
Again, here Jesus assures us that He is always with us, to the very end.
So Christ says His Church will not be destroyed or fall away from him, that the Holy Spirit will guide the Church to always teach the Truth, and He will always be with us. The Church is much more than a collection of nice people who read a Bible and believe anything they want.
What did Peter write?
"16For we did not follow cleverly devised myths when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. 17For when he received honor and glory from God the Father, and the voice was borne to him by the Majestic Glory, "This is my beloved Son, with whom I am well pleased," 18we ourselves heard this very voice borne from heaven, for we were with him on the holy mountain. 19And we have something more sure, the prophetic word, to which you will do well to pay attention as to a lamp shining in a dark place, until the day dawns and the morning star rises in your hearts, 20knowing this first of all, that no prophecy of Scripture comes from someone’s own interpretation. 21For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit."
And why? Because, in the next verse:
"1But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction. 2And many will follow their sensuality, and because of them the way of truth will be blasphemed. 3And in their greed they will exploit you with false words. Their condemnation from long ago is not idle, and their destruction is not asleep."
What is our guard against false prophets and teachers? Scripture. What will try to lead us astray? Man.
"In this case, it is Christ’s promise that the visible Church he established will always teach the Truth."
No. It is the function of the church to uphold the truth - not to replace it. And who - not what - teaches truth?
"26But 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 have said to you...7Nevertheless, I tell you the truth: it is to your advantage that I go away, for if I do not go away, the Helper will not come to you. But if I go, I will send him to you. 8 And when he comes, he will convict the world concerning sin and righteousness and judgment: 9concerning sin, because they do not believe in me; 10 concerning righteousness, because I go to the Father, and you will see me no longer; 11 concerning judgment, because the ruler of this world is judged.
12"I still have many things to say to you, but you cannot bear them now. 13When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come. 14He will glorify me, for he will take what is mine and declare it to you. 15 All that the Father has is mine; therefore I said that he will take what is mine and declare it to you."
Even the great apologists of the Catholic church, like G.K. Chesterton and Hillaire Belloc, did not think that the R.C. Church was acting as the pillar and foundation of the truth in the years leading up to the Reformation.