Galileo quotes Augustine many times in his letter and each quote sufficiently supports the point that Galileo is trying to make. Considering that Galileo is basing his letter on the "truth," it can be said that Augustine is viewed almost as the "truth" itself in the eyes of the people Galileo is writing for. What better way to reveal the "truth," than by using the "truth" in his letter?.
So the reasons for Galileo's use of Augustine being established, it can also be seen in his letter how Augustine specifically supports his theories. The basis of both Galileo's and Augustine's views is the fact that the Bible should not be translated literally in matters of physical mysteries (184-185). Of course the central physical mystery that Galileo explores is the debate between the geocentric universe and the heliocentric universe. Apparently, the various leaders of the church and philosophers read the Bible literally, and from their reading they concluded that the earth was fixed in space, with the sun revolving around it (geocentric). This is also grounded in the Ptolemaic system of the universe, a system that many were reluctant to abandon (177). The geocentric theory clearly opposes the heliocentric theory, for the heliocentric theory explains that the earth rotates around the sun. So, Galileo's acceptance of the heliocentric theory was clearly against what the Church had decided was true. .
Even so, Galileo contends that the Bible can never speak untruth (181). It is heresy to believe differently. The Bible is full of mysteries and there is know way to begin to interpret everything that the Bible puts forth. Galileo quotes Augustine to prove his point: ". . . not the meaning of the Bible, which is beyond comprehension, but rather his own interpretation; not what is in the Bible, but what he has found in himself and imagines to be there" (186). So it can be gathered that both Galileo and Augustine do not believe that any one man can explain the secrets of the Holy Scriptures.