You would risk becoming a disbeliever if you call another Muslim a kāfir (disbeliever)
BUKHARI 6104:
from ʿAbdullāh b. ʿUmar —
The Messenger of Allah ﷺ said, "If anyone says to his brother O kāfir (disbeliever)! Then surely, one of them such."
البخاري ٦١٠٤:
عن عبد الله بن عمر —
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ: يَا كَافِرُ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا۔»