Respected, Sir. Shahji,
Kafir is an Arabic term exclusively used for a person who openly rejects or disbelieves in the existence of Allah, denies His dominion and authority. According to some Imams, rightfully or wrongfully, a person is a kafir who does not follow the basic tenets of Islam. What are the basic tenets of Islam? Do I need to explain?
Murtad (apostate) is the one who abandons Islam verbally as well as through deeds. Also, it includes the act of converting to another religion, non-acceptance of faith by a person born in a Muslim family or who had previously accepted Islam.
A person who follows the most prevalent N2I (goes to Mazars, goes to Pirs or Babas falls into Mushriq category which is worse than a Kafir).
Also, if one belongs to a sect his future is in great jeopardy because the Prophet did not belong to any sect.
Therefore, before calling anybody a Kafir or Murtad one has to make sure where he or she would stand on the day of judgement.