Sincerity and Sincere Advice
A superb explanation of the ḥadīth of Tamīm al-Dārī concerning naṣīḥah. This short excerpt covers all the finer points of how to be sincere to Allāh, His Book, His Messenger (ṣallallāhu ʿalayhi wa-sallam), the leaders of the Muslims, and the common-folk.
As regards sincerity to His Book, then it is to believe that it is the Speech of Allāh – the Most High – and His Revelation.
Tamīm al-Dārī (raḍī Allāhu ʿʿʿanhu) said:
“The Prophet (ṣallallāhu ʿalayhi wa-sallam) said, (three times), “The Religion is Naṣīḥah (sincerity and sincere advice).” We said, “To whom?” He said, “To Allāh, His Book, His Messenger, and to the leaders of the Muslims and the general people.” 
COMMENTARY UPON THE ḤADĪTH:
Imām al-Nawawī (d.676H) – raḥimahullāh– said:
“As regards sincerity to Allāh – the Most High – then its meaning refers back to having īmān (faith) in Allāh, and the denial of ilhaad  with regards to His Attributes. Descriibng Him – the Most High – with the qualities of completeness, perfection, and greatness whilst declaring Him completely free from any defects or deficiencies. To avoid disobedience to Him. To love for His sake and to hate for His sake. To keep good relations wit those who obey Him and to have enmity towards those who disobey Him. To recognize His favours and to give thanks to Him for them. To have sincerity in all affairs. To call to everything that we have mentioned, and to encourage it. To show kindness to all people, to all those whom you are able to, in this call. Imām al-Khaṭṭābī (d.388H) – raḥimahullāh– said, “The reality of this attachment refers back to the servant being sincere to himself, since Allāh – the Most High – has no need of the sincerity of the one doing it.”
As regards sincerity to His Book, then it is to believe that it is the Speech of Allāh – the Most High – and His Revelation. Nothing resembles it from the speech of the creation. No one from the creation is able to match it. Then, to give it due respect by reciting it and acting upon it as it should be done, and to do that well. To have khushūʿ’ (humbleness and submissiveness) when doing so. To correctly recite its letters. To defend it from the misinterpretation of the people who make taḥrīf (corruption and changes) and from the onslaught of those who attack it. To believe in what is in it. To affirm its rulings. That you learn its sciences and examples. That you take heed over its warnings and ponder over its amazing matters. That you act upon those things that are clear, and that you submit to those that are not so clear. That you seek after those matters which are general and specific, abrogated and abrogating. To promulgate its sciences and to call to it, all of this being sincerity to it.
As regards sincerity to the Messenger of Allāh, then it is to testify to the truth that he was sent with. To obey him in what he ordered and prohibited. To aid him and to assist him in life and in death. To be at enmity with those who are his enemies. To align oneself with those who have allegiance to him. To respect his rights and his honour. To revive his way and his Sunnah. To spread his daʿwah (call), and his sharīʿah (law). To expel any doubt that are raised against it. To give one's full attention to the sciences of Ḥadīth. To gain understanding of its meaning, and to call to it. To have mildness and kindness in learning and teaching it. To give due place and importance to it. To display correct manners when reading it. To withhold from speaking about it without due knowledge. To give due respect to its people because of their attachment to it. To take on board its manners and conduct. To love the Ahl al-Bayt (family of the Prophet), and his Companions. To avoid those who introduce bidʿah (innovations) into his Sunnah, and to avoid those who attack even a single one of his Companions.
Sincerity to the Leaders is to help them upon the truth. To obey them in it. To order them with it. To remind and advise them with kindness and gentleness. To remind them of that which they are heedless and neglectful of. To help them fulfill those rights of the Muslims that have not reached them yet. Not to rebel against them. To enamour the hearts of the people with obedience to them. Imām al-Khaṭṭābī - raḥimahullāh – says, “From sincerity to them is Prayer behind them, Jihād along with them, to give the zakāt (charity) to them, and not to rebel against them with the sword when injustice or bad treatment appears from them. And that they are not praised with false praises, and that duʿāʾʾʾ (supplication) is made for their righteousness.” All of this is based upon the fact that what is meant by the leader of the Muslims is the Caliph, and other than him from the administrators who take charge of the affairs of the Muslims. Imām al-Khaṭṭābī also quotes it, then says, “And it has also been taken to refer to the Imāms – who are the scholars of the Religion. And that sincerity to them includes accepting what they report, following them as regards rulings, and to have good thoughts about them.”
Sincerity to the general Muslims, and they are those who are other than the rulers and the scholars, it is to guide them to what is beneficial for them, both in the Hereafter and in this life. To keep harm away from them so that he teaches them that which they are ignorant of as regards the Religion. To help them in this by words and actions. To hide their faults and to fulfill their needs and wants. To remove that which is harmful to them, and to bring that which is of benefit to them. To order them with good and to forbid them from evil, with gentleness, sincerity, and compassion for them. Having respect for their elderly, and respect for their young. To give them good admonition, not acting deceitfully towards them. To love the good things for them, which he would love for himself. To hate the bad things for them, which he would hate for himself. To protect their wealth, and reputation, and other than that, with sayings and actions. To encourage them to take on the character of all that we have mentioned, from all the types of sincerity. And to enliven their feeling for acts of obedience. And there were some from the Salaf-–may Allāh be pleased with them all – whose advice was so great, that it caused severe harm to his worldly affairs – and Allāh knows best.” 
 Related by Muslim (no. 55)
 Denying or altering the meanings of the Names and Attributes of Allāh.
 Sharḥ Ṣaḥīḥ Muslim (2/38)