Save before doing a slider move.
Move slider to Narrowminded.
Hope that you get the event "Heresy", and it targets a province in the Americas.
Incite that province to revolt somehow.
Surrender to the religious rebels once they take a province.
FYI, you don't actually have to mess with the Americas. I didn't when I got this achievement. This will get you there a little easier:
Conquer and convert everything except one Shiite province (conquer it, but don't convert yet).
Place Missionary in Shiite province, reduce Missionary upkeep to zero.
Wait until Shiite province revolts, let rebels take province.
Accede to their demands (converts you to Shiite Islam)
Place Missionary in Sunni province that neighbors Shiite province.
Wait until Sunni province revolts, attack rebels with small force and drive them into Shiite province.
Let rebels take Shiite province (automatically converts it to Sunni).
Achievement.