Virtue of Tawheed (Oneness of Allah or Monotheism in Islam).

From the second chapter of Shaykh 'Abd ar-Rahman as-Sa'di's sharh of 'Kitab at-Tawhid':
"...There is nothing that has a better effect and more virtues than Tawhid, as all of the good in this world and the next sprout from this Tawhid and its virtues.
1 - The forgiveness and expiation of sins are from its virtues and effects.
2 - It is the greatest means by which the pressures, hardships, and punishments of this life and the next are repelled.

3 - It prevents one from remaining eternally in the fires of Hell, even if there was only a mustard seed’s worth of it in the heart. If it is complete in the heart, it prevents one from ever entering Hell in the first place!
4 - It grants its owner complete guidance and security in this life and the next.
5 - It is the only means of attaining the Pleasure and reward of Allah, and he who will be happiest in regards to the intercession of the Prophet is he who said ‘there is none worthy of worship except Allah’ sincerely from his heart.
6 - And from the greatest of its virtues is that the acceptance, richness, and reward of all actions and words – outer and inner – depend on Tawhid. So, as strong as Tawhid and sincerity to Allah are, the stronger these will become.
7 - It causes good actions and works to be much easier for the servant, just as it makes it easier for him to abandon bad and evil actions. It also keeps him preoccupied when a crisis occurs. So, acts of obedience are very light for the one who is sincere to Allah in his faith and Tawhid, due to what he hopes for of the reward and Pleasure of his Lord. Likewise, it is very easy for him to leave off the sins that his soul desires, due to his fear of Allah’s Anger and punishment.
8 - If it becomes complete in the heart, Allah causes its owner to love faith, and will beautify it in his heart. Likewise, he will make him hate disbelief, sin, and disobedience, will make him of those who are rightly guided.
9 - It makes unpleasant things easy for the servant, and will ease his pains. So, as much as the servant has complete Tawhid and faith, his heart will absorb unpleasant and painful things with openness, tranquility, submission, and satisfaction with the Decree of Allah.
10 - And from the greatest of its virtues is that it frees the servant from enslavement and attachment to, fear of, and hope in the Creation, as well as doing anything for their sake, and this is the true essence of honor and nobility. With this, he becomes truly enslaved to his Lord, does not hope for any but Him, does not fear any but Him, and does not depend on any but Him. With this, he has achieved true success and victory.
11 - And from its virtues that cannot be claimed by any other is that, if it is complete in the heart, and is truly and fully established with complete sincerity, it can turn small actions into huge ones, and multiplies actions and words without any effort whatsoever…and this is all because of the sincerity that is in his heart. And how many of those who say it do not reach such a level, and this is because his heart does not contain the level of complete Tawhid and sincerity – or anywhere near it – that such a person has?
12 - Allah has taken it upon Himself to bestow victory, honor, nobility, means to guidance, means of attaining relaxation, rectification of affairs, and firmness in speech and action to its people in this world.
13 - Allah defends the monotheists and people of faith from the various evils of this world and the next, blesses them with a pure life of feeling secure with Him and remembrance of Him.
And the proofs for this from the Book and Sunnah are many, are well-known, and Allah Knows best."
['al-Qawl as-Sadid Sharh Kitab at-Tawhid'; p. 12-13]