The following is taken from Robert Eno, "Doctrinal Authority in Saint Augustine," Augustinian Studies 12 (1981): 166-67:
At the end of his life, Augustine admitted
uncertainty as to how the "Tu es Petrus" should be interpreted.
Sometimes, he had said that Peter was the rock; more often he had commented
that Christ was the rock and the Church was built on him whom Peter confessed. [115]
The reader, he concluded in the Retractations, was free to choose. More often,
for Augustine, the strength and firmness of Christ the rock were to be
emphasized as the Church's one foundation. "The rock was Christ."
"I shall build my Church on myself, the Son of the living God. I shall
build you upon myself, not myself on you ... " "That Peter may be
built up on the rock, not the rock on Peter." "Peter is named from
the rock, not the rock from Peter, just as the Christian takes his name from
Christ, not vice versa." [116]
Notes
for the Above
115. Retract. 1.21.1 (CSEL 3612.98). Christ
is the rock: Tr.in ev. Joann. 124.5 (CCL 36.684-5); Sermo 270.2
(pL 38.1239).
116. "The rock is Christ.": En.in ps.60.3
(CCL 39.767); Church built by Christ on himself: Sermo 76.1 (PL 38.479;
see also: sermo 244.1 (PL 38.1148); En.in ps.102.9 (CCL
40.1459-60); en.in ps.103, sermo 2.5 (CCL 40.1493); "Peter
is named from the rock": Sermo 270.2 and Tr.in ev.Joann. 124.5.
See note 115 above