Karl Barth spends a few pages at the very beginning of his Church Dogmatics insisting that theology has a right to consider itself to be a science. Of course he does not mean an experimental science, which is more or less the only definition of science that we have in English. Nor does he intend that theology should be forced to conform to the norms and methodologies of a general 'science', whatever those might look like. Theology must do its own thing, but it is (or can be) none the less scientific.
I would suggest that there is one key reason why we should consider theology a science, and that is that it has a definite object. Theology is not speculative - or at least, in so far as it is speculative, it is bad theology. Theology is the investigation of a particular object, namely God. As a discipline, it is bound to this object. It examines and describes, but it does not invent.
Now God is never merely the object of our investigations; God is never merely an object at all. Like any other person, God is also subject. Indeed, as divine Person, God is always subject, and always sovereign subject, in all of his interactions with anything outside himself. But he gives himself as an object. He allows himself to be investigated. He makes himself available as the source and object of theological science. He does not give himself away - he is very capable, for example, of recalling the errant theologian back to his truth. He remains the sovereign subject. But he is also there, there for us to see and investigate and learn.
To put it in theological terms, God in the incarnation has come to us. Because he was here, as a human being amongst human beings, there is a definite historical referent behind our theological talk. We cannot just say anything about God, as if he were a mysterious noumena, without shape or form or limit. God gives himself as object in this particular person at this particular time. Therefore, he can be known; therefore, there is theology. Because of Jesus, theology is a science.
The theological method will be decided by its object. Because its object is God as he gives himself to us in Jesus, its method must be the study and exposition of the witness to Jesus contained in Holy Scripture. Because its object is the God who establishes the church as his community of witness, its method must be community based. Because its object is the the God who is also always subject, its method must be driven by prayer and worship.
But it is not speculative or open-ended.
Because of Christmas, theology has definite content, just as the manger of Bethlehem had definite content.