Holmes claims that "The early church claimed that all truth is God's truth
wherever it be found" (p. 8). He also quotes Clement of Alexandria and
Justin Martyr (p. 14), but neither Justin nor Clement actually use the exact
phrase.
It appears that Holmes believes that the phrase is obviously true and
obviously Christian, but it is not a phrase that he literally attributes to
any other previous thinker.
I am indebted to my colleague Randy Basinger, another fine Christian
philosopher, for the information in this post.
Ted Davis