One can reason oneself into monotheism. Everything has a cause, the the cause has a cause, etc, etc. Eventually, this loop has to terminate somewhere. Aristotle got his idea of the Prime Mover, but otherwise his writing is almost that of an atheist, clearly he had not considered God as a Person. The doctrine of his teacher, Plato, is a bit more intriguing. While Socrates in Platonic dialogues pays homage to Greek gods, in his reasoning it becomes evident he's a monotheist who believes in a single Creator, who is also fundamentally good. Although, Plato is rather evidently not fond of the world and his ideal Republic is at war with it, lest the evils of the imperfect world corrupt the citizens of the ideal polis. There were other pagan monotheists, like the Stoics, but, unlike Platonic-Aristotlean thought, they haven't influenced the Christian philosophy and theology much, so their ideas fell on wayside. But all of this is just human philosophy, no divine revelation. That's where Faith comes in; Reason alone can bring you no further.
And here's a passage from the Acts (17:15-33):
"And they that conducted Paul brought him unto Athens: and receiving a commandment unto Silas and Timotheus for to come to him with all speed, they departed; Now while Paul waited for them at Athens, his spirit was stirred in him, when he saw the city wholly given to idolatry Therefore disputed he in the synagogue with the Jews, and with the devout persons, and in the market daily with them that met with him.Then certain philosophers of the Epicureans, and of the Stoicks, encountered him. And some said, What will this babbler say? other some, He seemeth to be a setter forth of strange gods: because he preached unto them Jesus, and the resurrection.And they took him, and brought him unto Areopagus, saying, May we know what this new doctrine, whereof thou speakest, is? For thou bringest certain strange things to our ears: we would know therefore what these things mean. (For all the Athenians and strangers which were there spent their time in nothing else, but either to tell, or to hear some new thing). Then Paul stood in the midst of Mars' hill, and said, Ye men of Athens, I perceive that in all things ye are too superstitious. For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you. God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands; Neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things; And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation; That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us: For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring. Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device. And the times of this ignorance God winked at; but now commandeth all men every where to repent; Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead. And when they heard of the resurrection of the dead, some mocked: and others said, We will hear thee again of this matter. So Paul departed from among them."
So, St. Paul clearly considers the Athenians to be honoring the same God as he does, but lacking revelation, which is Christ Jesus.
