Yankel Berel said – The fact is that one could be a bona fide religious zionist nowadays without any agadic heresy either.
Agree with you 100%, although it may depend on the specific flavor of ‘Dati Leumi’ in question. The DL world itself is a broad and colorful spectrum from people who are Chareidi plus in their lifestyle and עבודת השם, but also believe that building settlements on the hills of Shomron will be מקרב the גאולה, to people like Naftali Bennett, who I wouldn’t take לכתחילה as an עד for anything, but not because of the ‘Zionist’ stuff…
However, it is also perfectly possible for someone to be fully observant of Orthodox Judaism, at least outwardly, and still be an אפיקורוס, only based on his krum beliefs or lack thereof. Just to give an easy example: One of the 13 Ikkarim is belief in תחיית המתים. There is no נפקא מינה in day-to-day behavior or observance of other areas of Halacha directly connected to that belief. One could even theoretically believe in שכר ועונש and עולם הבא (in some form other than what our Mesorah says), yet לכל הדעות failure to believe in this particular doctrine (which you may call אגדתא), clearly triggers Halachic consequences and makes a person a bona-fide אפיקורוס who is פסול לעדות etc. There is precedent in Poskim (at least לכתחילה) to avoid taking as an עד someone who clearly denies or is מזלזל in beliefs, minhagim, or aspects of the Mesorah that have been accepted by כלל ישראל, even if he doesn’t technically deny any of the 13 Ikkarim or violate any Halacha.