Determinism refers to pre-existing causes. What will be will be, because causes and conditions that have "gone" before have set forth cause and effect chains that determine the end. Secular determinism is the framework of Godless science (not that science is necessarily godless). Theological determinism is the framework of Calvinism.
By way of illustration, let's say there is a person who sees the probability of an unpleasant future looming ahead of her. She decides to take action and change the course of her future. She takes what action she can, and experiences a chain of events she would not have if she had not taken action. It appeared she had changed the course of affairs and the initial probability wasn't so probable anymore. Unfortunately, the thing she feared came upon her anyway.
A fatalist would say, "I told you so, there is nothing that can be done to avert or change what is fated." The outcome proves the premise. What would the religious determinist say about that end that would be different in any useful way? The thought to avert the future was predetermined; the course of mitigation attempted was predetermined; the apparent success of that course was predetermined; and the ironic result of the whole affair was predetermined. How is the analysis of the end result any different for one viewpoint as opposed to the other?
If a theological determinist is to be a determinist, that one is also, ultimately, a fatalist. Whether one looks at an end occurring regardless of intervening actions, or examines consequent actions step by step, the result is the same--what ends up happening happens because it was necessary and it could not have occurred otherwise. That one cares more about the steps to get there than the other, seems to me to make precious little useful difference, in the end, at all.