One issue of disagreement between Roman Catholics (RC) on one side and Eastern Orthodox and Protestants (EO&P) on the other is the interpretation of Mt 16:18. Literally translated from Aramaic into English, the 1st half of Mt 16:18 would be:
"And so I say to you, you are Rock, and upon this rock I will build my church". (Mt 16:18)
Where both instances of "rock" were "kepha" in the original Aramaic, in which the same word kepha is used for a rock or a man's name. When the text was translated to Greek (by Matthew or the final author), as the word "petra" for a rock is feminine, the 1st Kepha was changed to "Petros" (masculine ending) to make it suitable for a man's name.
The issue of disagreement between the RC and EO&P interpretations comes from Jesus' implicit body language. I will make it explicit between () to render the two interpretations of Mt 16:18:
"And so I say to you (pointing to Simon), you are Rock, and (still pointing to Simon) upon this rock I will build my church". (RC interpretation)
"And so I say to you (pointing to Simon), you are Rock, and (now pointing to Himself) upon this rock I will build my church". (EO&P interpretation)
To note, the EO&P interpretation is conceptually in line with 1 Peter 2:4-5, which uses "lithos" = "stone" instead of "petra" = "rock":
"Come to Him (Jesus), a living stone, rejected by human beings but chosen and precious in the sight of God, and, like living stones (the faithful), let yourselves be built into a spiritual house to be a holy priesthood to offer spiritual sacrifices acceptable to God through Jesus Christ." (1 Peter 2:4-5)
Thus, in the EO&P interpretation Jesus is the rock / cornerstone upon which the Church is built, and Simon is just another living rock / stone in the Church's building, just like any other faithful is.
Actually, the EO&P interpretation is correct in a certain sense: at the personal, spiritual, existential level Jesus is the rock upon which we the faithful are built like living stones (the same rock referred to many times in the Psalms, which is logical since Jesus is "I Am", YHWH), and this truth applies to Simon, his successors the bishops of Rome, and any other faithful. The problem is that it is not logical to interpret Jesus' words in Mt 16:18 in that sense, for several reasons:
1. Why would Jesus rename Simon as Rock only to say immediately that He Himself was the rock?
2. More importantly, why would Jesus rename Simon as Rock if He was meaning that Simon was just another living rock in the Church's building?
3. Even more importantly, the only way to interpret adequately Mt 16:18 is by reading it within the whole statement by Jesus. Let's see what the EO&P interpretation looks like:
17: Jesus said to him in reply, "Blessed are you, Simon son of Jonah. For flesh and blood has not revealed this to you, but my heavenly Father. (In this verse Simon is definitely special.)
18: And so I say to you, you are Rock, and upon this rock (Myself) I will build my church, and the gates of the netherworld shall not prevail against it. (In this verse Simon is just like any other faithful, even though he is renamed for some strange reason.)
19: I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven. (In this verse Simon is again special. And thanks to the fact that in Greek personal pronouns have differerent forms for singular and plural, it cannot be argued that there was yet another change in Jesus' body language and now He is pointing to the circle around Him so that "you" refers now to the 12 Apostles.)
We must use reason to interpret the Scripture. An interpretation of Mt 16:18 where "upon this rock" does not refer to Simon defies all logic.