How can one reconcile the fact that a person is not held accountable for sins of the heart unless they become firmly established and continuous, with the saying of some scholars that whoever resolves to commit disbelief has disbelieved immediately?

A passing thought or whisper [of the soul] is not held against a person; rather, one is rewarded for abandoning it if it is evil. As for resolve (al-azm), it is an action of the heart for which its doer is held accountable.

Faith in the heart includes affirmation and certainty (the 'saying' of the heart), and submission and love (the 'action' of the heart). Disbelief occurs with the loss of either of these two types.

What the jurists mean by 'settled action' is that which is not merely a fleeting whisper that a person repels.

Whoever intends disbelief or resolves upon it disbelieves immediately, because this intention or resolve is an independent action of the heart that contradicts faith and negates the required continuity in intention.