St. Augustine, revered by Christians and heretics alike as a great Christian thinker and Church Father was very well read when it came to philosophy. Many times did he quote the philosophers in his writings, sometimes agreeing with them and at other times showing the errors in their thinking, and always doing so to prop up and defend some truth of the Catholic religion.
St. Augustine, City of God, Book 8, Chapter 1: "We shall require to apply our mind with far greater intensity to the present question than was requisite in the solution and unfolding of the questions handled in the preceding books; for it is not with ordinary men, but with philosophers that we must confer concerning the theology which they call natural.
"For it is not like the fabulous, that is, the theatrical; nor the civil, that is, the urban theology: the one of which displays the crimes of the gods, while the other manifests their criminal desires, which demonstrate them to be rather malign demons than gods. It is, we say, with philosophers we have to confer with respect to this theology,— men whose very name, if rendered into Latin, signifies those who profess the love of wisdom.
"Now, if wisdom is God, who made all things, as is attested by the divine authority and truth, (Wisdom 7:24-27) then the philosopher is a lover of God. But since the thing itself, which is called by this name, exists not in all who glory in the name—for it does not follow, of course, that all who are called philosophers are lovers of true wisdom—we must needs select from the number of those with whose opinions we have been able to acquaint ourselves by reading, some with whom we may not unworthily engage in the treatment of this question.
"For I have not in this work undertaken to refute all the vain opinions of the philosophers, but only such as pertain to theology, which Greek word we understand to mean an account or explanation of the divine nature. Nor, again, have I undertaken to refute all the vain theological opinions of all the philosophers, but only of such of them as, agreeing in the belief that there is a divine nature, and that this divine nature is concerned about human affairs, do nevertheless deny that the worship of the one unchangeable God is sufficient for the obtaining of a blessed life after death, as well as at the present time; and hold that, in order to obtain that life, many gods, created, indeed, and appointed to their several spheres by that one God, are to be worshipped.
"These approach nearer to the truth than even Varro; for, while he saw no difficulty in extending natural theology in its entirety even to the world and the soul of the world, these acknowledge God as existing above all that is of the nature of soul, and as the Creator not only of this visible world, which is often called heaven and earth, but also of every soul whatsoever, and as Him who gives blessedness to the rational soul—of which kind is the human soul—by participation in His own unchangeable and incorporeal light.
"There is no one, who has even a slender knowledge of these things, who does not know of the Platonic philosophers, who derive their name from their master Plato. Concerning this Plato, then, I will briefly state such things as I deem necessary to the present question, mentioning beforehand those who preceded him in time in the same department of literature."
Those who would condemn St.Thomas Aquinas or the hierarchy of the Church for using philosophy, then, might as well (which God forbid!) condemn St. Paul himself for saying the following:
1st Corinthians 9: 22-23: "To the weak I became weak, that I might gain the weak. I became all things to all men, that I might save all. And I do all things for the gospel's sake: that I may be made partaker thereof."
And they might as well condemn St. Augustine for following in St. Paul's footsteps. God forbid!
St. Augustine, Letter 118 3:16: "Among those, again, who say that our supreme and only good is to enjoy God, by whom both we ourselves and all things were made, the most eminent have been the Platonists, who not unreasonably judged it to belong to their duty to confute the Stoics and Epicureans— the latter especially, and almost exclusively."
But study of the philosophers must be balanced along with study of theology, as the decree from the Council of Lateran V states:
Pope Leo X, Fifth Lateran Council, Session 8: "And since truth cannot contradict truth, we define that every statement contrary to the enlightened truth of the faith is totally false and we strictly forbid teaching otherwise to be permitted. We decree that all those who cling to erroneous statements of this kind, thus sowing heresies which are wholly condemned, should be avoided in every way and punished as detestable and odious heretics and infidels who are undermining the Catholic Faith.
"Moreover we strictly enjoin on each and every philosopher who teaches publicly in the universities or elsewhere, that when they explain or address to their audience the principles or conclusions of philosophers, where these are known to deviate from the true faith—as in the assertion of the soul's mortality or of there being only one soul or of the eternity of the world and other topics of this kind—they are obliged to devote their every effort to clarify for their listeners the truth of the Christian religion, to teach it by convincing arguments, so far as this is possible, and to apply themselves to the full extent of their energies to refuting and disposing of the philosophers' opposing arguments, since all the solutions are available.
"But it does not suffice occasionally to clip the roots of the brambles, if the ground is not dug deeply so as to check them beginning again to multiply, and if there are not removed their seeds and root causes from which they grow so easily. That is why, since the prolonged study of human philosophy—which God has made empty and foolish, as the Apostle says, when that study lacks the flavouring of divine wisdom and the light of revealed truth—sometimes leads to error rather than to the discovery of the truth, we ordain and rule by this salutary constitution, in order to suppress all occasions of falling into error with respect to the matters referred to above, that from this time onwards none of those in sacred orders, whether religious or seculars or others so committed, when they follow courses in universities or other public institutions, may devote themselves to the study of philosophy or poetry for longer than five years after the study of grammar and dialectic, without their giving some time to the study of theology or pontifical law. Once these five years are past, if someone wishes to sweat over such studies, he may do so only if at the same time, or in some other way, he actively devotes himself to theology or the sacred canons; so that the Lord's priests may find the means, in these holy and useful occupations, for cleansing and healing the infected sources of philosophy and poetry."