On the conventional account, theology differs from other forms of reflection, including religious studies, for one or both of two reasons: (1) because it has to appeal to special criteria of truth for some if not all of its assertions; and (2) because the theologian has to be a believer who already holds these assertions to be true. (Ogden)
People of the Christian faith cannot accept principles of other religions, unless they conform to Christianity. An example would be a Christian believes Jesus Christ is the Savior and is equal to God the Creator. This means a Christian would not adopt views contrary to this, such the Islamic or Judeo belief in one God. Theology only reaffirms the belief in Jesus Christ.
different from religious studies, as well as from other forms of reflection, even it it were in full compliance with the same standards of reflection, since the sufficient ground of its difference is the question that constitutes it a distinct field of reflection—namely, the reflective question as to the meaning and truth of the Christian religion, or witness of faith. (Ogden)
“Religious Studies" is often used as a general heading, covering a multiplicity of possibly conflicting methodological approaches to religion. Some of them still tend to be regarded as alternatives to "Theology". (Leirvick)
Many of the non-Christian religions have religious instruction in their Temples, Mosques, and other institutions instead of teaching their religion in colleges. College courses for these religions are for knowledge, not the believer. Theology is taught at colleges and universities built around the theological study. Religious studies are for knowledge, whereas theology is for the believer in Christianity.
Theology is the study of the Christian faith, whereas religious study deal with every other religion. Theology is taught in mainstream colleges and universities. Religious studies are also taught in mainstream colleges and universities,