If there is no God then everything came in to existence through naturalism. This is the philosophy of many evolutionists and where their presuppositions come from. To say scientists do not possess presuppositions is nonsense.
"If there is no God" is assuredly NOT a proper topic within science. Science is not about whether or not there is a Deity or Deities: it is about observation and testing of the physical(natural) world.
There is no such thing as 'an evolutionist': there are people who find the ToE the most comprehensive and accurate explanation for speciation, and the best description o the means by which it came about. That would be nearly all people with education and training in scientific method and various 'physical' sciences.
Your unverifiable speculations on the 'philosophy' you attribute to 'many' such scientists remains simply your single opinion. I am unable to calculate 'many' as a percentage, nor have you described your 'methodology' to arrive at that 'figure'..... (ie, Have you sent out questionnaires? How large was your sample, and how did you select the respondents?)
To say that scientists give free rein to 'presuppositions' *in their field of study* is a peculiarly insulting and demeaning assertion - particularly when there is no objective, verifiable, reproducible way to analyze the situation and attempt to arrive at any accurate result.