This is your opinion, that happens to not accord with the scripture. There was no scripture at the Pentecost. Whatever the Rock of Peter means, Christ surely did not speak of the scripture as the foundation of the Church either.
Once more, I read what is written and you theorize about it.
Annalex. Have you read Peter's sermon at Pentacost? If you had (and retained any of it) you could not say there was no Scripture at Pentacost. Just for a refresher...
Acts 2
4But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:
15For these are not drunken, as ye suppose, seeing it is but the third hour of the day.
16But this is that which was spoken by the prophet Joel;[In Joel 2, which is in SCRIPTURE]
17And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams:
18And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy:
19And I will shew wonders in heaven above, and signs in the earth beneath; blood, and fire, and vapour of smoke:
20The sun shall be turned into darkness, and the moon into blood, before the great and notable day of the Lord come:
21And it shall come to pass, that whosoever shall call on the name of the Lord shall be saved. [end of SCRIPTURAL QUOTE at Pentacost #1]
22Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
23Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:
24Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.
[Beginning of Scriptural Quote #2]
25For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:
26Therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope:
27Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption.
28Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance. [End of Scriptural Quotation from Psalm 16]
29Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day.
[Beginning of Scriptural Quote # 3 - from Psalm 132]
30Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne; [End of Scriptural Quote #3]
31He seeing this before spake of the resurrection of Christ, that[Revisitation of Scriptural Quote #2] his soul was not left in hell, neither his flesh did see corruption.[End of Scriptural revisitation]
32This Jesus hath God raised up, whereof we all are witnesses.
33Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear.
34For David is not ascended into the heavens: but he saith himself, [Beginning of Scriptural Quotation #4] The Lord said unto my Lord, Sit thou on my right hand,
35Until I make thy foes thy footstool.[End of Quote #4 from Psalm 110 and the contention that there was NO SCRIPTURE AT PENTACOST]
Now, why did Peter quote Scripture at Pentacost? Because even he, an eyewitness to Christ's ministry and Apostle did not stand in his own authority but in the authority of the Word of God. The Church began with the foundation of Scripture. The Church grew in understanding with the foundation of Scripture. And, apart from the foundation of Scripture the Church has no standing. Peter found it necessary to speak using it as the foundation of his speech. Paul did the same. And Jesus HIMSELF gave us the very same example. Apart from Scripture, the Church has no standing. It is the foundation laid by the Christ and the key tool that the Spirit uses to teach us with.
Do not dismiss its importance and place the church above it. The church is not above the Word, but is subservient to it since it is the perfect revelation of God to man.
Jesus --->Holy Spirit--->Truth---->Scripture----->Church