So why would only that part be in Aramaic?
http://www.olivetree.com/cgi-bin/EnglishBible.htm
Matt 16:18 `And I yet to thee am saying, that thou art Peter, and upon this/
tauth <3778>, the rock/
petra <4073> I shall be building of Me the 0ut-called, and gates of Hades not shall be prevailing of her;
petra <4073> Used 4 times in NC/NT Matt 16:18, Matt 27:60, 1 Corin 10:4, 1 Peter 2:8
Hebrew 11:2 For in this/
tauth <3778> were testified to the Elders/presbuteroi 3 To Faith we are apprehending...........
'
Regarding Aramaic I am quoting a source for you and anyone else interested. There will be a link to the source at the end.
QUOTE
Aramaic in the New Testament
"We know that Jesus spoke Aramaic because some of his words are preserved for us in the Gospels. Look at Matthew 27:46, where he says from the cross,
Eli, Eli, lama sabachthani? That isnt Greek; its Aramaic, and it means, My God, my God, why have you forsaken me?
"Whats more," I said, "in Pauls epistlesfour times in Galatians and four times in 1 Corinthianswe have the Aramaic form of Simons new name preserved for us. In our English Bibles it comes out as
Cephas. That isnt Greek. Thats a transliteration of the Aramaic word
Kepha (rendered as
Kephas in its Hellenistic form).
"And what does
Kepha mean? It means a rock, the same as
petra. (It doesnt mean a little stone or a pebble. What Jesus said to Simon in Matthew 16:18 was this: You are
Kepha, and on this
kepha I will build my Church.
"When you understand what the Aramaic says, you see that Jesus was equating Simon and the rock; he wasnt contrasting them. We see this vividly in some modern English translations, which render the verse this way: You are Rock, and upon this rock I will build my church. In French one word,
pierre, has always been used both for Simons new name and for the rock."
For a few moments the missionary seemed stumped. It was obvious he had never heard such a rejoinder. His brow was knit in thought as he tried to come up with a counter. Then it occurred to him.
"Wait a second," he said. "If
kepha means the same as
petra, why dont we read in the Greek, You are
Petra, and on this
petra I will build my Church? Why, for Simons new name, does Matthew use a Greek word,
Petros, which means something quite different from
petra?"
"Because he had no choice," I said. "Greek and Aramaic have different grammatical structures. In Aramaic you can use
kepha in both places in Matthew 16:18. In Greek you encounter a problem arising from the fact that nouns take differing gender endings.
"You have masculine, feminine, and neuter nouns. The Greek word
petra is feminine. You can use it in the second half of Matthew 16:18 without any trouble. But you cant use it as Simons new name, because you cant give a man a feminine nameat least back then you couldnt. You have to change the ending of the noun to make it masculine. When you do that, you get
Petros, which was an already-existing word meaning rock.
"I admit thats an imperfect rendering of the Aramaic; you lose part of the play on words. In English, where we have Peter and rock, you lose all of it. But thats the best you can do in Greek."
Beyond the grammatical evidence, the structure of the narrative does not allow for a downplaying of Peters role in the Church. Look at the way Matthew 16:15-19 is structured. After Peter gives a confession about the identity of Jesus, the Lord does the same in return for Peter. Jesus does not say, "Blessed are you, Simon Bar-Jona! For flesh and blood has not revealed this to you, but my Father who is in heaven. And I tell you, you are an insignificant pebble and on this rock I will build my Church. . . . I will give you the keys of the kingdom of heaven." Jesus is giving Peter a three-fold blessing, including the gift of the keys to the kingdom, not undermining his authority. To say that Jesus is downplaying Peter flies in the face of the context. Jesus is installing Peter as a form of chief steward or prime minister under the King of Kings by giving him the keys to the kingdom. As can be seen in Isaiah 22:22, kings in the Old Testament appointed a chief steward to serve under them in a position of great authority to rule over the inhabitants of the kingdom. Jesus quotes almost verbatum from this passage in Isaiah, and so it is clear what he has in mind. He is raising Peter up as a father figure to the household of faith (Is. 22:21), to lead them and guide the flock (John 21:15-17). This authority of the prime minister under the king was passed on from one man to another down through the ages by the giving of the keys, which were worn on the shoulder as a sign of authority. Likewise, the authority of Peter has been passed down for 2000 years by means of the papacy.
http://www.catholic.com/library/Peter_the_Rock.asp