What is the concept of sincerity in the Qur'an?

“Sincerity“, literally, has the meaning of being pure and unadulterated. The concept of sincerity mentioned in the Qur’an denotes turning to Allah in devoutness. A sincere believer devotes all his deeds throughout his entire life to seeking the pleasure of Allah and expects his reward only from Him. He does not make devious calculations orientated around the opinions of other people or trying to find favour with them. Therefore all of his actions are sincere and candid, such as would earn the pleasure of Allah.

A sincere person is honest first with Allah and then with people; because he well knows that Allah hears and sees all things, and that he will be judged in His presence and will be held responsible for all of his deeds, words and thoughts. For this reason the sincerity and candour of believers are among the most significant signs of the firmness of their faith. Allah has made the sincere attitudes of the messengers, who always sought to earn the good pleasure of Allah, an example for believers:

And remember Our servants Ibrahim, Ishaq and Ya’qub, men of true strength and inner sight. We purified their sincerity through sincere remembrance of the Abode. In Our eyes they are among the best of chosen men. (Surah Sâd, 45-47)

Those who turn to Allah in sincerity are praised in the Qur’an. The Qur’an heralds the good outcome of these people as follows:

… Those who shun the worship of false gods and turn towards Allah will have good news. So give good news to My servants. Those who listen well to what is said and follow the best of it, they are the ones whom Allah has guided, they are the people of intelligence. (Surat az-Zumar, 17-18)

What is the concept of sincerity in the Qur'an?