Two very useful thoughts are always borne in the mind of a Believer:
One, I don’t know whether my good deeds have been accepted or not.
Two, I don’t know how my life would end up (i.e. would it be a good-ending or a bad one).
By contemplating on these thoughts we could always refrain from being complacent regarding our efforts on the Path of Allah.
This would InshaAllah help us improve our state of Iman (Faith) and A’maal (Good Deeds). We could save ourselves from being boastful of accomplishing abundant good deeds. Whenever we feel some satisfaction due to accomplishment of abundant good deeds, right at that moment we must think that iblees engaged in much more worship, but what good had all those resulted for him finally?
Derived from an excerpt of Arba’yeen of Imam Gazzali (R)