It is ok to believe a god started the Big Bang and science can't disprove the untestable.
Until science finds out what started the Big Bang. Then which gaps fill the theists next with their god?
This has happened over and over throughout history. And always they have said, but scientists will never explain X, only god could have done that. And then science explained X. And theists burned the scientists with fire (maybe sometimes literally)literally) until they had to change their position because the world threatened to move on without them.
It will happen again...
That debate would become an endless loop. Hypothetically science finds out what triggered the big bang and the faithful will simply claim that god created that trigger. And so on ad nausea.
I say give them this so long as their beliefs don't contradict estabished and proven science.