I know exactly what a scientific theory is, but that's irrelevant to anything we're talking about. Basically my point is that since scientific theories exist which use the word "believe", but aren't politicised, and most likely never will be, I don't think politicisation is what causes the use of the word "believe".
The word "believe" in common usage often has very little to do with the concept of a "belief", in terms of faith i.e. the 5th definition
here. In scientific articles and seminars, "believe" is kind of the default word choice when something makes sense according to the current understanding but you can't be bothered to verify it with additional experiments, e.g. "We believe this anomalous result is due to uncontrolled temperature fluctuations." It's kind of a weaker synonym for "this is most likely due to". But since it's harder to say "most likely due to" when your theory isn't too sound, the word "believe" pops up more with less substantiated theories, regardless of politics/faith etc. Less substantiated theories are also easier to politicise e.g. you don't see too much politicisation of the Young Earth theory since there's overwhelming evidence otherwise, but evolution is still politicised.
In other words, politicisation and the use of the word "believe" aren't directly linked causally, they're both correlated to how substantiated a scientific theory is.
Sorry for derailment.