@anesotericmuslim: We often feel sincere in a moment, but sincerity isn't proven in a moment, it's proven over time. Even those close enough to Moses & the Prophet (pbuh) to write down revelation, like Abdullah ibn Sa'd ibn Abi Sarh, could falter; even those given knowledge and signs, like Balaam, lost their way when they let go of it. So what about us? That's why we don't just ask for sincerity, but for a sincerity that lasts, a heart that stays firm in both hardship and ease and when we fall again, because we will, we don't lean on our deeds, we hold onto His mercy, asking Him not to let this sincerity be temporary. (Source The Seal of The Prophets Volume I & II)