In the Peshitta Syriac New Testament the Greek word “PETRA” is translated by the Aramaic word SHU`A’ as in Matthew 7:24-25 meaning a massive rock or a boulder.
PETRA is used 16 times in the Greek New Testament:
Of those times it is translated in the Peshitta Syriac
9 times by the word SHU`A’ ,
6 times by the word KE’PHA’ and
1 time by the Hebrew root word ‘ABENA’
Of the ten times PETRA is used in the Gospels it is translated:
7 times by the word SHU`A’
(Mt.7:24, 25; Mk.15:46; Lk 6:48[2x];8:6, 13)
3 times by the word KE’PHA’
(Mt.16:18; 27:51; 27:60)
Of the three times KE’PHA’ is used to translate PETRA in the Gospels:
 in Mt. 27:60 the parallel passage in Mark’s gospel (Mark 15:46) more correctly uses SHU`A’ to translate PETRA.
 in Mt. 27:51 the word KE’PHA’ is used to describe the rocks (plural) which were broken at the earthquake when Christ died (and hence, these rocks became movable).
 the other passage is Mt. 16:18 where KE’PHA’ is used to translate both PETROS and PETRA.
In all other places in the Gospels the Greek word PETRA is translated by the Syriac word SHU`A’, meaning “a massive rock.”
KE’PHA’ is used in the Syriac N.T. as the translation of both the Greek words LITHOS and PETROS.
The Greek word LITHOS, which means “a stone” (generally of a size which could be picked up or moved) is ALWAYS translated by the Syriac word KE’PHA’.
As LITHOS in classical Greek is the common prose word for “a stone” (see the quote from Liddle and Scott’s Lexicon, above) and PETROS is more common in poetry, this shows that the definition of KE’PHA’ as “a stone” is correct. The Syriac KE’PHA’ is equivalent to the Greek LITHOS, a movable stone.
KE’PHA’ IS ALWAYS USED TO TRANSLATE THE GREEK WORD LITHOS.
SHU’A IS THE MORE USUAL AND CORRECT SYRIAC WORD TO TRANSLATE THE GREEK WORD PETRA.
KE’PHA IS A MOVABLE STONE = LITHOS / PETROS.
SHU’A IS A MASSIVE ROCK = PETRA.
The Syriac word SHU
A' is NEVER used to translate the Greek word LITHOS. Because a LITHOS is NOT a large massive rock, but a SHUA’ is. The Syriac KE’PHA’ is correctly used to translate the Greek words LITHOS and PETROS because these are movable stones.
The fact that the Greek text of the New Testament uses two separate Greek words in the passage [Matthew 16:18] indicates that any underlying Aramaic/Syriac original also must have used two separate words.
a. A reconstructed Aramaic/Syriac of the passage would properly be: “You are KE’PHA’ (a movable stone) and upon this SHU`A’ (a large massive rock) I will build my church.”
This is in exact correspondence to the original inspired Greek text: “You are PETROS (a movable stone) and upon this PETRA (a large massive rock) I will build my church.”
b. The Peshitta Syriac New Testament text, at least in its extant Manuscripts, mistranslated the passage in Matthew 16:18, incorrectly using the Syriac word KE’PHA’ for both Greek words PETROS and PETRA.
c. The Church of Rome bases its doctrine of Peter being the Rock upon which the Church is built on this mistranslation and/or a falsely reconstructed Aramaic/Syriac original, ignoring the distinctions in the Aramaic language.