today:
Iran's population is majority Shiite but Sunni minorities live predominantly in the areas near the border with Afghanistan. The Sunnis have long complained about discrimination by Iranian authorities. Due to dilapidated infrastructure and a lack of health and educational facilities, the areas near the Afghan border are the poorest and least developed in Iran.
“The Taliban's leadership structures are multilayered, complicated and opaque. That makes negotiations with them difficult, and not just for Iran.”
https://www.dw.com/en/why-iran-fears-chaos-in-afghanistan/a-59630081
Big Al antedates the big old Mo by nearly 1000 years, iow, there was no Islam up in there, or anywhere. Seems likely that there was no dominant religion there, although Persian Zoroastrianism may have existed in the garrison towns and such, along with pre-Zoroastrian religious practices that included Mithraism. Buddhism started early enough that it may have been present here and there, but it wouldn't have been dominant (in India, Buddhism died out by the 12th c) and may have been pretty much invisible.
There was no Islam of any kind, when Alexander the Great blew through there. Islam didn’t appear until the 600s.