"He is the Rock, his work is perfect: for all his ways are judgment: a God of truth and without iniquity, just and right is he." - Deuteronomy 32:4

"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." - Matthew 16:18

(The Rock upon which the eternal church/congregation of the saints of GOD is built is The Lord Jesus Christ against Whom/which the gates of hell will never prevail because everyone in that church/congregation who are in Christ Jesus The Lord, by the mercy of GOD, has been justified & has mercifully received GOD's pardon through the atoning sacrifice of The Lord Jesus Christ on the cross / from the foundation of the world, bearing our sins and iniquities on Himself). ---> "And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone." (Ephesians 2:20, 1 Peter 2:6). Let GOD be magnified!

"And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ." - 1 Corinthians 10:4

Thus, in Matthew 16:18, The Lord Jesus Christ is referring to Himself/GOD, The Rock, The only Savior (Acts 4:12).  "I, even I, am the LORD; and beside me there is no saviour." - Isaiah 43:11, (and Isaiah 45:21). GOD is One GOD.

[ Much like this part of Holy Scripture - 1 Timothy 3:15
"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 ground of the truth." The "Pillar and Ground of Truth" is GOD / The Lord Jesus Christ, "the Way, the Truth & the Life" - John 14:6 and not the local/earthly churches and congregations, (evidenced as early as the church in Corinth and the churches in Asia minor ref: to in Revelations 2 & 3, etc), though they've served God's purpose. ]

"And why call ye me, Lord, Lord, and do not the things which I say? Whosoever cometh to me, and heareth my sayings, and doeth them, I will shew you to whom he is like: He is like a man which built an house, and digged deep, and laid the foundation on a rock: and when the flood arose, the stream beat vehemently upon that house, and could not shake it: for it was founded upon a rock." - Luke 6:46-48 (also, Matthew 7:24-25)

Living Waters:
"Behold, I will stand before thee there upon the rock in Horeb; and thou shalt smite the rock, and there shall come water out of it, that the people may drink. And Moses did so in the sight of the elders of Israel." - Exodus 17:6, (Nehemiah 9:15, Psalm 78:15-16, Psalm 105:41, Isaiah 48:21) - (symbolically portrayed, yet substantively, the Rock from which living water came / comes is The Lord Jesus Christ, our GOD upon whom we depend on and believe/trust in for salvation / eternal life, from Whom we receive living water unto eternal life).

"Jesus answered and said unto her, Whosoever drinketh of this water shall thirst again: But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life." - John 4:13-15

GOD, The Lord Jesus Christ, from whom living waters flow:
"For my people have committed two evils; they have forsaken me the fountain of living waters, and hewed them out cisterns, broken cisterns, that can hold no water." - Jeremiah 17:13

"Jesus answered and said unto her, If thou knewest the gift of God, and who it is that saith to thee, Give me to drink; thou wouldest have asked of him, and he would have given thee living water." - John 4:10

"He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water." - John 7:38

"For the Lamb which is in the midst of the throne shall feed them, and shall lead them unto living fountains of waters: and God shall wipe away all tears from their eyes." - Revelation 7:17]

Again, The Rock:
"There is none holy as the LORD: for there is none beside thee: neither is there any rock like our God. Talk no more so exceeding proudly; let not arrogancy come out of your mouth: for the LORD is a God of knowledge, and by him actions are weighed." - 1 Samuel 2:2-3

"And David spake unto the LORD the words of this song in the day that the LORD had delivered him out of the hand of all his enemies, and out of the hand of Saul: And he said, The LORD is my rock, and my fortress, and my deliverer; The God of my rock; in him will I trust: he is my shield, and the horn of my salvation, my high tower, and my refuge, my saviour; thou savest me from violence." - 2 Samuel 22: 1-3

"Truly my soul waiteth upon God: from him cometh my salvation.  He only is my rock and my salvation; he is my defence; I shall not be greatly moved… My soul, wait thou only upon God; for my expectation is from him. He only is my rock and my salvation: he is my defence; I shall not be moved. In God is my salvation and my glory: the rock of my strength, and my refuge, is in God." - Psalm 62: 1-2 & 5-7

"As for God, his way is perfect; the word of the LORD is tried: he is a buckler to all them that trust in him. For who is God, save the LORD? and who is a rock, save our God?" - 2 Samuel 22:31-32

"To shew that the LORD is upright: he is my rock, and there is no unrighteousness in him." - Psalm 92:15

"The LORD is my rock, and my fortress, and my deliverer; my God, my strength, in whom I will trust; my buckler, and the horn of my salvation, and my high tower." - Psalm 18:2

"The LORD liveth; and blessed be my rock; and let the God of my salvation be exalted." - Psalm 18:46 (Psalm 28:1, Psalm 31:2-3, Psalm 42:9, Psalm 71:3, 78:35, Psalm 94:22, Psalm 95:1, Isaiah 17:10 )

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Isaiah 45:22-24
"Look unto me, and be ye saved, all the ends of the earth: for I am God, and there is none else. I have sworn by myself, the word is gone out of my mouth in righteousness, and shall not return, That unto me every knee shall bow, every tongue shall swear. Surely, shall one say, in the LORD have I righteousness and strength: even to him shall men come; and all that are incensed against him shall be ashamed."

Philippians 2:9-11
"Wherefore God also hath highly exalted him, and given him a name which is above every name: That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father."

Psalm 89:20-37 (in actuality David, but symbolically and substantively The Lord Jesus Christ)
"[b]I have found David my servant; with my holy oil have I anointed him: With whom my hand shall be established: mine arm also shall strengthen him. The enemy shall not exact upon him; nor the son of wickedness afflict him. And I will beat down his foes before his face, and plague them that hate him. But my faithfulness and my mercy shall be with him: and in my name shall his horn be exalted. I will set his hand also in the sea, and his right hand in the rivers.  He shall cry unto me, Thou art my father, my God, and the rock of my salvation. Also I will make him my firstborn, higher than the kings of the earth. My mercy will I keep for him for evermore, and my covenant shall stand fast with him.  His seed also will I make to endure for ever, and his throne as the days of heaven.  If his children forsake my law, and walk not in my judgments; If they break my statutes, and keep not my commandments; Then will I visit their transgression with the rod, and their iniquity with stripes. Nevertheless my lovingkindness will I not utterly take from him, nor suffer my faithfulness to fail.  My covenant will I not break, nor alter the thing that is gone out of my lips. Once have I sworn by my holiness that I will not lie unto David.  His seed shall endure for ever, and his throne as the sun before me.  It shall be established for ever as the moon, and as a faithful witness in heaven. Selah.[/b]"