Quran 5:72 – Difference Between Kufr And Kafir?
– That person cannot be called a Mushrik unless he admits it but we can point to his practices as having shirk in them with arguments. – Trinity is a shirk and effectively kufr however, to call people believing in trinity as Mushrik or Kafir, we need evidence of their admission and their deliberate denial of monotheism.
Please also refer to the video below from 3:28 to 4:40 https://youtu.be/8toBwUoqsjI?si=478Dea7U8S39MTth&t=208