Mubtila means someone who has been overtaken or is fallen, being tested.
Here is a couplet from Momin’s famous ghazal ‘woh jo hum mein tum mein qaraar tha’
jise aap kehte the baa-vafaa jise aap ginte the aashna
main wahi hoon Momin-e-mubtalaa, tumhein yaad ho ke naa yaad ho
The one you called loyal, the one you counted as your friend
I am that same fallen Momin, maybe you remember or maybe you don’t