It is not so much that they weren't known of at all. At least the big names, like Plato and Aristotle were widely known by reputation, and read through a handful of translations (particularly Boethius's translations of Aristotle's organon and Calcidius's translation of Plato's Timaeus) in at least some periods. It is more that they didn't broadly have a culture of reading Greek. The classics that were read were the standard latin school texts, particularly Vergil but also Horace, Lucan, Sallust and Cicero to list the four most common that come down to us in manuscript form. As a result, there wasn't systematic collection of Greek manuscripts or use of Greek authors, except when latin translations were available. Interestingly, of latin authors, Origen, in translations of his work produced by Jerome and Rufinus, was actually one of the more widely read, particularly in the early middle ages.

The fundamental issue stems back to the fundamental breakdown of Greek culture in the west by the end of fifth century. In general, the Roman elite was educated in both Greek and Latin, and hence we have few classical translations into Latin as there was little need. There are a few exceptions, like Cicero's translation of a small portion of the Timaeus, but anyways I will leave the issue of Greek and Latin in Antique education to a classicist to deal with. There was a brief period in the west around the beginning of the sixth century where certain groups, most notably Boethius, were interest in trying to preserve Greek authors through translation. But Boethius himself was executed before he could translate more than a handful of Aristotles works, completing most of the logical works except the posterior analytics, as well as Porphyry's Isagoge (an introductory textbook to Aristotle's Categories) and there just wasn't a sufficient interest in these translations at the time for others to maintain that tradition.

On on the point of influence. Though Greek philosophy was certainly broadly influential on Christianity, it is important to remember that Augustine's view, at least, was that Christianity was the perfection of Greek philosophy, indeed arguing, for example, that when Plato went to Egypt he must have been taught by the prophet Jeremiah. And though Augustine and other important fathers supported the use of classical learning, their use and endorsement of Greek authors wasn't unqualified and didn't suggest that reading these authors directly was a Christian imperative.

There were some exceptions. For example, John Scotus Eriugena (c. 815 – c. 877) certainly read greek, using particularly Maximus the Confessor in some of his philosophical works, and he produced some translations, notably of Pseudo-Dionysius. But there just wasn't much interest in Greek culture and therefore Greek authors generally between the sixth and twelfth centuries. And it wasn't until the redevelopment of widespread and interconnected higher education in the twelfth century that the conditions were in place to produce translations of major Greek, well mostly Aristotle, and Greek influenced Arabic authors. Finally there wasn't an interest in actually teaching Greek in the west until about the fifteenth/sixteenth century.

Edit: proofreading...