Philosophy is related to theology like your right hand is to your left hand.
Philosophy is based on the light of human reason. So in philosophy we can ask questions like, “What is the world made of?”, “Did the world always exist or was it created?”, “How does one thing differ from another?”, “What is existence?”, “What is human nature?”, “What is the purpose of life and how do we fulfill our lives?”, “What is truth?”, “What is knowledge?”, “What is happiness and how do we find it?”, “How is it best to live our lives?”, “How should we organize our societies?”, “What is beauty? How do you define the beautiful?”, and so on. Philosophers try to sketch an answer to such questions using their own reason, and to reason and deliberate and dialogue with other philosophers in the attempt to approach the truth of these things more closely.
Theology is based on the light of faith, on faith in the Revelation of God—the event of God revealing Himself. Through the Catholic faith, for example, we know that God seeks out human beings and wants to have a relationship with them, to lead them to eternal happiness. We know that God sent His only Son, Jesus Christ into the world to show us the nature of God, that God is Trinity, that Christ died for our sins and rose again from the dead, that He founded a Church, that He sent His Holy Spirit upon the Church for our salvation and sanctification and that of the whole world, and so on. Theologians seek to deepen their understanding of such truths as these which have been revealed by God and received by the Church in faith.
From this we can see that, while each has its own point of departure, philosophy (based on reason) and theology (based on faith) overlap in their fundamental concerns and, at the same time, that both disciplines are oriented to the truth about existence, human nature, and human destiny.
Moreover, each of philosophy and theology involves the cooperation of faith and reason: philosophy embraces certain premises (or beliefs) and proceeds to reason accordingly; theology believes certain precepts and proceeds to seek an understanding of them (using reason).
For all these reasons, philosophy and theology are marked not only by cooperation and complementarity; they are also, in some sense, inseparable.
We may say, therefore, that philosophy and theology are (1) one in origin (their ultimate origin is in God who gives us both the light of reason and the light of faith), (2) one in their goal (they are both oriented to truth), and (3) one in their operation (or cooperation), in the sense that they are quite inseparable.