The term is presupposition.
presupposition noun
1 A thing tacitly assumed beforehand at the beginning of a line of argument or course of action.
‘both men shared certain ethical presuppositions about the universe’
-Lexico
In any discussion, each party will come with their own presuppositions.
In a case where an evolutionist has a discussion with a creationist, the creationist comes with the presupposition that scripture is true and sacrosanct while the evolutionist comes with the presupposition that evolution is true and sacrosanct.
The parties might have examined their own presuppositions deeply, or they might simply take them on faith. The creationist might have looked into global features such as rapid water-based erosion that have left 'tide marks' such as those in the Grand Canyon, while the evolutionist might have looked into historical eyewitness accounts of canyon formation.
However, at some point, presuppositions will be left unexamined. The creationist might not be able to read the original languages, so needs to form views based on the translations of those who claim they can. The evolutionist might not have personally observed sexual reproduction where the child is of a different species to that of both parents, so needs to form views based on the records of those who claim they have.
Even this question and answer presupposes that you and I hold enough of the English language in common that what we understand of each other's posts bears some resemblance to the communicative intent of the respective poster.
In a sense, both parties begin with their conclusions. They may change their presuppositions if they find a sufficiently persuasive argument, but no logical conversation can even begin if both parties don't start with some presupposition. The term presupposition also tends to be applied to more deeply-held notions (such as exclusive materialism or divine involvement), which is why it can feel as if each party begins with their conclusions.