"Assuming that" is a useful synonym for the phrase "taking for granted that." This phrase implies that you are making an assumption without examining the evidence or considering other possibilities. Other synonyms could include "presuming," "supposing," or "accepting without question." All of these phrases share the same tone of confidence that a certain situation or fact is true, without any need for further justification. While it is sometimes appropriate to make assumptions in a conversation or argument, it is also important to recognize when more evidence is needed to support a claim. Using synonyms for "taking for granted that" helps to avoid making unfounded conclusions.