Kafir simply means the denier. Now, you openly say you dont believe Hazrat Mirza Ghulam Ahmad AS , that makes you kafir. However, because you are the follower of Muhammad (PBUH), the master of Hazrat Mirza Ghulam Ahmad (AS), and you do believe in Muhammad PBUH , this makes you Muslim but kafir of Promised Messiah (AS).
I am squeezing the discussion. I am not re-writing the stuff again for you which i have already written. If you like you can scroll back to read again.
however i want to add one thing; which might create confusion in your mind to understand better.
I dont agree with this; "if i dont believe in Mirza I am kafir but at the same time i believe in Prophet Muhammad (PBUH), i am muslim". This does not make any sense to me.
What i understand; If Mirza's claim is true, and i dont believe in him, i will be kafir, even if i believe in Prophet Muhammad and tawheed of Allah.
If you dont take it this way which i am saying; Less precedence will make belief in Promised Mehdi un-important. A person can remain muslim even if he dont believe in Promised Mehdi.
I am not taking the term kafir like the poet took in his phrase, which is correct but i am not taking it like that. poet says;
"Hum Kafiroun kay Kafir hain, Kafir Khuda Humara."
I am taking it differently. which is also correct. that is as follows.
A Kafir "denier" of one true prophet of Allah is kafir of all prophets, and any one who accepts a false prophet as true is also denier "kafir" of all prophets and Allah.
I have read (dont remember where, plz dont ask to provide the reference) the same concept in your literature also; when Mirza Qadiyani says; whoever denying Promised Mehdi actually denying Prophet Muhammad (PBUH), similarly whoever denies Prophet Muhammad (PBUH) denies Allah (SAW).
Using the literal mean of term "Kafir of'" in this sense is simply cheating with people and non-sense.
Believe in Promised Mehdi is so important that it is just a boundry line between kufr and Islam.