Q 9: What does "Sincerity of determination" mean?
A--- It means warding off lethargy and laziness, and exerting one's effort in making his words go with his deeds.
Allah the Almighty says: {O you who believe! Why do you say that which you do not do? Most hateful it is with Allah that you say that which you do not do}.
(As-Saff,2-3)
Reference: 200 FAQ On Muslims Belief - Shiekh Hafiz Ibn Ahmed Al Hakami