Non-Christian prophets may be cited : De Civitate Dei BOOK 18

from Gerard O'Daly (2020, 2nd edition) page 220

Chapter on Books 15 - 18 of Augustine's The 

City of God THE READER's EDITION (Oxford

Univ. Press):

Just as the LXX translators are prophets,

so too there are non-Jewish prophets,

whom Christians may cite.  For every if

there were no people of God other than

the Jews, there were individuals who were

citizens of the heavenly City.  JOB,

presented by Augustine as neither a native

of Israel nor a proselyte (Latin nec indigena

nec proselytus) is one such example.  These

individuals (Sibylline oracles/ seers, possible

among others) are prophets only as the 

result of a divine revelation.  Their faith is

one and the same as that of Christian

believers (De Civitate Dei Book 18 #47).

Yet in the Christian Church as it is 

constituted, there are those who are not

true members (Book 18 #48).  Many

unworthy members are mixed with the good,

caught in the Gospel's dragnet, and swimming

in this word as in a sea (Book 18 #23),

before the separation of the evil from the

good.  This a consequence of the great

increases in the number of Christians.  To

these thoughts Augustine appends a further

_instalment of his historical survey, summar-

izing events of Christ's life (Apostles, preaching,

death, resurrection, post-resurrection period

with the disciples, ascension), the coming of

the Holy Spirit, the spread of the Gospel,

persecutions and martyrdom of early Christians

(Book 18 #49-50).  The end of this process

is the Christianization of emperors who succeed

those who persecuted Christians and persecute

paganism in their turn (Book 18 #50).