What was the significance of Jesus' instructions to the disciples at Transfiguration?

What was the significance of Jesus' instructions to the disciples at Transfiguration?

Answer : No one can say Jesus is Lord without the Holy Spirit.The Apostles must all wait first for the Holy Spirit to come, and filled them with all the gifts needed to proclaim and testify to the Truth about Jesus Christ. So, that all the people will believe them, because the "Spirit of Truth" is the one speaking thru them and guiding them in all Truth.

12“I have many more things to say to you, but you cannot bear them now. 13“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. 14“He will glorify Me, for He will take of Mine and will disclose it to you. 15“All things that the Father has are Mine; therefore I said that He takes of Mine and will disclose it to you.-John16:12-14

The simple answer biblically was revealed by St.Paul the Apostle in 1Corinthian12:3,

Spiritual Gifts …3Therefore I inform you that no one who is speaking by the Spirit of God says, “Jesus be cursed,” and no one can say, “Jesus is Lord,” except by the Holy Spirit. 4 There are different gifts, but the same Spirit. 5There are different ministries, but the same Lord.…

Jesus instructed the Apostles not to reveal it yet, because, no one can preach and testify that "Jesus is Lord" without having the anointing and gifts of the Holy Spirit. In witnessing for the Truth about who Jesus Christ is, one needs to be "filled by the Holy Spirit", as we can see this with St.Elizabeth, proclaiming the "Jesus is Lord", because a Virgin Priest or like a Bishop, pours out the gifts of the Holy Spirit, resembling the "mysterious greetings" of St.Gabriel, pouring down all the gifts, making Her not just filled with the Holy Spirit, but "full of graces".

Mary Visits Elizabeth …42In a loud voice she exclaimed, “Blessed are you among women, and blessed is the fruit of your womb! 43And why am I so honored, that the mother of my Lord should come to me? 44For as soon as the sound of your greeting reached my ears, the baby in my womb leaped for joy.… Luke1:43

Catholic Church teaches, that all graces flows thru the Maternal Hands of Mary, and no one can say Jesus is Lord, without experiencing the role of Mary in their lives, as St.Elizabeth experienced it, and all the Apostles experienced it too, as Mary prepared all their hearts, to receive all the gifts and graces of the Holy Spirit at the Upper Room, and ofcourse, thru the Maternal Hands of Mary.

Mary is the Mediatrix of all graces, and no one can say and preach "Jesus is Lord", if they ignore, the role of Mary as Mediatrix of all graces. All christians, who preach "Jesus is Lord" without acknowledging the Blessed Virgin Mary, is proclaiming a fake jesus, according to their personal interpretations and aspirations only.

62. This maternity of Mary in the order of grace began with the consent which she gave in faith at the Annunciation and which she sustained without wavering beneath the cross, and lasts until the eternal fulfillment of all the elect. Taken up to heaven she did not lay aside this salvific duty, but by her constant intercession continued to bring us the gifts of eternal salvation.(15*) By her maternal charity, she cares for the brethren of her Son, who still journey on earth surrounded by dangers and cultics, until they are led into the happiness of their true home. Therefore the Blessed Virgin is invoked by the Church under the titles of Advocate, Auxiliatrix, Adjutrix, and Mediatrix.(16*) This, however, is to be so understood that it neither takes away from nor adds anything to the dignity and efficaciousness of Christ the one Mediator.(17*)- Lumen Gentium62

Source :https://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19641121_lumen-gentium_en.html

The Catholic Church, as a rule, is going to follow the Fathers, especially when there is consensus.

St. Hilary of Poitiers:

He enjoins silence respecting what they had seen, for this reason, that when they should be filled with the Holy Spirit, they should then become witnesses of these spiritual deeds.

St. Jerome:

So he does not want this to be told to the people in case it should be deemed incredible because of its greatness and also so that after such great glory the event of the cross that follows should not cause untaught minds to stumble.

St. John Chrysostom:

For the greater the things said about him, the harder it was for the many at that time to accept them. And the offense of the cross increased all the more thereby. Therefore he told them to be silent about the transfiguration. He again reminded them of the Passion, and he almost mentioned the reason why he told them to be silent. For he did not command them never to tell anyone but “[to wait] until he is raised from the dead.” Saying nothing about the painful part, he told them only of the good. What then? Were they not going to be offended after this? Not at all. For the silence that was being demanded was only for the time before the crucifixion. After this they were thought better prepared to receive the Spirit. They had the voice of the miracles advocating for them, and everything that they said from then on was easier to receive. For the course of events announced his power more clearly than a trumpet, and no stumbling block interrupted these events.

George Leo Haydock:

Tell the vision to no man, till the miracle of his resurrection has prepared the minds of men for the belief of this. Expose not an event so wonderful to the rash censure of the envious Pharisees, who calumniate and misrepresent my most evident miracles. Jesus Christ also gave a lesson here to his followers to observe the closest secrecy in all spiritual graces and favors

https://catenabible.com/mt/17