We Muslims are ready to label kafir to the Muslim of other sects without realizing that it is a great sin in the eyes of Allah. Islam was completed in the days of Prophet Muhammad (PBUH) as Koran says:
اَلْیَوْمَ اَکْمَلْتُ لَکُمْ دِیۡنَکُمْ وَاَتْمَمْتُ عَلَیۡکُمْ نِعْمَتِیۡ وَرَضِیۡتُ لَکُمُ...