As explained in the notes, the two Greek words petros and petra are quite distinct, the former being masculine gender, and the latter feminine. The latter denotes a rock or cliff, in situ, firm and immovable. The former denotes a fragment of it, which one traveler may move with his foot in one direction and another may throw in another. This former word petros is the Greek translation of kephas, a stone, which was Peter's name in Aramaic, as was his appellative "Bar-jona" (John 1:42). See Appendix 94. III. 3.
  It is remarkable that there is only one other instance (Luke 22:34) in which our Lord addressed him as "Peter"; but, in all other cases, by his fore-name "Simon", reminding him of what he was before his call, and of the characteristics of his human nature. In that other instance it is used in connection with the coming exhibition of his weakness, in the prediction of his denial of his Lord.
  There is thus a special significance in the use of the word "Peter" in Matthew 16:18. It was the name connected with his commission and apostleship; another commission being about to be committed to him.

  It was not Peter, the man, who would be the foundation, for, as we have said, petra is feminine, and must refer to a feminine noun expressed or implied. That noun could hardly be any other than homologia, which means a confession; and it was Peter's confession that was the one subject of the Father's revelation and the Son's confirmation.
  Moreover, in 1Corinthians 3:11 it has once for all been declared by the Holy Spirit that "OTHER foundation can no man lay than that IS  LAID, which is JESUS CHRIST."

  The earliest known reference to Matthew 16:18 is found in ORIGEN'S Commentary (A.D. 186-253), which is older than any extant Greek manuscript. He says:

  "If we also say the same as Peter, 'Thou art the Christ, the Son of the living God', not by the instruction of flesh and blood, but by the illumination of the heavenly Father in our hearts, we ourselves become the same thing as Peter.
  "If you should think that the whole Church was built by God only on that one, Peter, what will you say of John, . . . or each of the apostles?" ¹

  This is conclusive as to the interpretation. But there are other and later references to these words by AUGUSTINE (A.D. 378), and JEROME (A.D. 305), alike older than any Greek Manuscripts now extant.
  JEROME wrote thus in his exposition (Benedictine ed.):

  "And I tell thee, that thou hast said to Me, 'Thou art the Christ', etc., and I tell thee that thou art Peter, and on this rock, etc." ²

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 ¹ ei de epi ton hena ekeinon Petron nomizeis hupo tou Theou okiodomeisthai ten pasan ekklesian monon, ti oun phesais peri Ioannou, tou tes brontes, e hekastou ton apostolon.
  ²  "Quid est quod ait? Et ego dico tibi tu mihi dixisti (tu es Christus filius Dei vivi); et ego dico tibi quia TU mihi dixisti (tu es Christus filius Dei vivi); et ego dico tibi (non sermone casso et nullum habenti opus, sed dico tibi, quia meum dixisse, fecisse est) quia tu es Petrus; et super hanc petram aedificabo ecclesiam meam."

  AUGUSTINE wrote in his Retractationes (Benedictine ed., vol. i, p. 33):

  "I have somewhere said, concerning the apostle Peter, that the Church was founded on him, as a petra, or rock; but I know that I have since very often explained what our Lord said to signify on Him Whom Peter confessed; but between these two opinions, let the reader choose that which is the more probable." ³

  In AUGUSTINE'S Sermon In die Pentecostis (Benedictine ed., tom. v. p. 1097; also Pusey's Translation, Sermons on the New Testament, vol. i. p. 215), he explains the reason for this retractation in a paraphrastic citation of the whole context:—

  "When our Lord had asked His disciples who men said that He was, and when, in reporting the opinions of others, they had said that some said He was John, some Elijah, others Jeremiah or one of the prophets, He said to them: 'But ye, Who do ye say that I am?' Peter (one alone for the rest, one for all) answered, 'Thou art the Christ, the Son of the living God.' This, most excellently, most truly spoken, was deservedly rewarded with this reply: 'Blessed art thou, Simon Bar-Jonah, because flesh and blood revealed not this to thee, but My Father Who is in heaven; and I tell thee that thou hast said': (hast said, observe, hast made confession unto Me: receive therefore the benediction): 'and I tell thee that thou art Peter; and on this rock I will build My church.'" ⁴

  Some have conjectured from these words "tu dixisti" (thou hast said it) that AUGUSTINE and JEROME must have had in the Manuscripts from which they translated six letters, which they divided into two words "SU  EIPS",⁵ taking EIPS as an abbreviation of EIPAS (= thou hast said).
  There must have been another division of the same six letters into three words, which was current even then, for both these Fathers add "SU  EI PETROS" = thou art Peter; taking the same "PS" as an abbreviation of PETROS.
  It is evident, however, that these Fathers give only a paraphrase; and do not profess to be giving an exact quotation.
  One thing, however, is certain, and that is our only point in this Appendix, namely, that the earliest references made to this passage disclaim all idea of its having any reference to the apostle Peter, but only to HIM Who was the subject of Peter's confession.
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  ³ "Dixi in quodam loco de apostolo Petro, quod in illo, quasi in petra, fundata sit ecclesia; sed scio me postea saepissime sic exposuisse quod a Domino dictum est, ut super hunc intelligetur quem confessus est Petrus: horum autem duarum sententiarum quae sit probabilior, eligat lector." (Italics, ours.)
  ⁴  "Cum interrogasset ipse Dominus discipulos suos, quis ab hominibus diceretur, et aliorum opiniones recolendo dixissent; quod alii eum dicerent Ioannem, alii Eliam, alii Ieremiam, aut unum ex prophetis, ait illis, 'Vos autem quem Me esse dictis?' Et Petrus, unus pro ceteris, unus pro omnibus, 'Tu es, inquit, Christus filius Dei vivi.' Hoc, optime, veracissime, merito tale responsum accipere meruit: 'Beatus es, Simon Bar Ionae, quia non tibi revelavit caro et sanguis, sed Pater Meus qui in coelis est: et Ego dico tibi, quia tu dixisti': Mihi dixisti audi; dedisti confessionem. Recipe benedictionem ergo: 'Et dico tibi, Tu es Petrus—et super hanc petram aedificabo ecclesiam Meam'".
  ⁵  It will be seen from Ap. 94. V. i. 3 that in the Greek manuscripts there was no division between the letters or words until the ninth century.