Intro
You will find Ahmadis all over social media claiming to be the one true sect of Islam.  They are yelling slogans, they are selling their books, clerics/employees are running around trying to get converts (for job security), and they never listen. They normally speak off of a memorized script.  One specific arguments has to do with a certain hadith wherein the hadith seems to claim that there will be 73 sects in Islam, and all of them, except one will go to hell.
This hadith states that there will be 73 sects of Muslims, and all of them will go hell except one.  Ahmadis claim to be that “one” and even tote themselves as the “true-muslims”.  This is Takfir, this is dangerous behavior.  As a Muslim who lives in America, I never question anyone’s right to be a Muslim.  However, Ahmadis have done it over and over and over, and refuse to admit to it.  Furthermore, Ahmadi’s don’t double check the sources and follow blindly based on faith and trust.

What did MGA write about this hadith?
In 1907, MGA wrote: 

“”””The truth of the matter is that after the passage of the first three centuries, the blessed ummah split into seventy-three sects and hundreds of mutually contradictory beliefs took root among them to the extent that they were no longer in agreement even regarding such beliefs as the appearance of the Mahdi and the coming of the Messiah. For example, the Mahdi of the Shias is hidden in a cave and he has the original Holy Quran. He will appear at a time when the Companions, Allah be pleased with them, will also be brought back to life, and he will take revenge from them for the usurpation of the Khilafat [from Hadrat ‘Ali ra].””””
(Haqiqatul Wahy, online english edition, see pages 54-55)

Some commentary
MGA didn’t specifically mention that his sect was the only one to go to heaven in this specific quotation, however, in other comments he does elude towards that.  This is MGA’s only comment on this hadith.  It seems that after 1974, Mirza Nasir Ahmad began to use this hadith more frequently and thus brainwashed a generation.

The founder of Wahabism used the same hadith
Feel free to look this up….Abdul Wahab used the same hadith to market his revolution

What does this hadith really mean?
Yasir Qadhi explains that this hadith is allegorical, its doesnt actually mean that there will be 73 sects of Muslims.  It only means that there will be lots of division, and the only Muslims who will go heaven will be the ones who mimic the Prophet (saw) and his companions.  And this is only for Allah to decide.

Ahmadis themselves have about 5-9 sects
Ahmadis are very ironic, since they themselves have had over 20 splinter sects in 100 years. How many Ahmadis from there will be sent to hell?  Will the entire Qadiani jamaat be sent to hell and the Lahori-Ahmadis are the saved sect??  This is ridiculous, Ahmadiyya-arguments are always self-contradictory and collapse easily.

MGA created a new sect in 1900
As we all know, MGA asked Ahmadis to be counted seperately in the 1900 census.  This is against the Quran, the Quran states:

006.159
Indeed, those who divide their religion and break up into sects, you have no part with them in the least: their affair is with God: He will in the end tell them the truth of all that they used to do”

Links and Related Essays

https://ahmadiyyafactcheckblog.com/?s=Takfir