How can a servant be insincere in his supplication to Allah, the Almighty and Majestic, when he turns to Him alone and to no one else?

Sincerity to Allah Almighty is a prerequisite for supplication. It means purifying one's humble plea to Allah alone, without seeking anyone else's pleasure. This is based on the supplicant's complete conviction that benefit and harm are in Allah's hands, and that He alone is worthy of supplication. Sincerity is contradicted by ignorance of its true meaning and of Allah's Oneness, such as resorting to intermediaries to seek intercession, or the heart's heedlessness of Allah's greatness and its attachment to worldly means, or due to the soul's desires for showing off (riya') and loving people's praise. Riya' has been condemned in a Hadith (definition) as a minor polytheism (shirk (definition) asghar). Therefore, the Sunnah (definition) in supplication is to conceal it rather than vocalize it, as Allah Almighty says: "(Call upon your Lord in humility and secretly. Indeed, He does not like aggressors)" Al-A'raf/55. Allah Almighty also praised His servant Zachariah for calling upon his Lord in a hidden call.