The word "theology" loosely used stands, as its etymology makes clear, for any knowledge of God. For philosophical purposes, it is necessary to distinguish three kinds of theology, one of which really is not theology at all, but often parades as if it were.
In the three religions of the West (Judaism, Christianity, and Islam) there is what should be called dogmatic or sacred theology -- scared because it is based on the revealed word of God in sacred scriptures, and dogmatic because it deal with a religion's articles of faith, dogmatically declared.
Such sacred and dogmatic theology is called by Thomas Aquinas the queen of the sciences, and philosophy is her handmaiden. The thinking that is done by the philosopher in scared theology is directed by religious faith; and the process by which sacred theology is developed can be described as faith seeking understanding. The role Thomas Aquinas played in Christian theology is played by Moses Maimonides in Jewish sacred theology, and by Avicenna in the sacred theology of Islam.
Philosophical theology, neither sacred not dogmatic, represents thinking about God that is done by pagans -- persons without religious faith, or at least by individuals who do not appeal to the articles of any religious faith. My book How to Think About God (1984) was written when I was still pagan. It was written for pagans. I must confess that the thinking I did in writing that book was influential in my becoming a Christian in 1984.
The third mode of the theology has often been called natural theology, but unlike philosophical theology, it is a body of purely natural knowledge with no appeals to dogmas or articles of faith. What is traditionally called natural theology is really apologetics, Christian, Jewish, or Islamic. It is a defense of the faith by persons who have that faith, and it is addressed to those who do not share that faith.
The Summa Theologica of Thomas Aquinas is a work in sacred theology. He wrote another series of volumes entitled Summa Contra Gentiles, addressed to the Moors and Jews in Spain, attempting to persuade them that the Christian faith he was defending is the one true faith, and should be adopted by them. That is a work in Christian apologetics.
The English bishop William Paley wrote a work in Christian apologetics, and so too did John Locke. The titles of their books are, respectively, A View of the Evidence of Christianity (1794) and The Reasonableness of Christianity (1731).
The literature on this subject would be clarified if such works were acknowledged as Christian apologetics, instead of claiming to be natural theology. The latter term should be reserved for works in philosophical theology that should always be pagan in their character for both writers and readers.