How can Tawheed be achieved and what is thepromised reward?

How can a person achieve Tawheed or truebelief in the Oneness of Allaah?.
Praise be to Allaah.
Praise be to Allaah and blessings and peace be upon the Messenger of Allaah.
You have asked – may Allaah bless you – about an important matter,
which is easy for the onefor whom Allaah makes it easy. We ask Allaah
to make it easy for us and for our Muslim brothers to attain all that
is good.
It should be noted that achieving Tawheed or true belief in the
Oneness of Allaah can only be done by testifying truly that there is
no god but Allaah and that Muhammad is the Messenger of Allaah.
Achieving this is of two degrees, one that is obligatory and one that
is mustahabb.
The obligatory degree is achieved by means of three things:
1- Giving up shirk (association of others with Allaah) in all its
forms, major, minor and concealed.
2- Giving up bid'ah (innovation) in all its forms.
3- Giving up sin in all its forms.
The mustahabb degree is that in which people may vary greatly, and it
means not having anything in the heart of attachment to anything or
anyone other than Allaah, so the heart is focused entirely on Allaah
and pays no attention to anything or anyone else; he speaks only for
the sake of Allaah and his deeds andactions are all for Allaah and all
his thoughts are focused Allaah, may He be glorified and exalted.Some
scholars described this degree as: giving upsomething that is
permissible so as to avoid something that is forbidden; that includes
actions of the heart, tongue and physical faculties.
In order to achieve thesetwo degrees, certain things are essential:
1 – Knowledge, otherwise how can one attain Tawheed or true belief in
the Oneness of Allaah and act upon it if he does not know of it or
understand it? Each accountable adult must learn about the Oneness of
Allaah that which will make his beliefs, words and deeds correct, then
anything more than thatis a bonus.
2 – Firm, certain and deeply-rooted belief in that which was
narratedfrom Allaah, may He be glorified and exalted and His Prophet
(peace and blessings of Allaah be upon him) of reports and words.
3 – Obedience to the commands of Allaah and His Messenger (peace and
blessings of Allaah be upon him) by doing that which is enjoined, and
avoiding that whichis forbidden.
The more a person achieves these things, the stronger will be his
Tawheed and the greater will be his reward.
Our Messenger (peace and blessings of Allaah be upon him) has
explained to us that the one who attains the highest level of
Tawheedis the one who is promised that he will be with the seventy
thousand who will enterParadise without being brought to account –
weask Allaah of His bounty.
In Saheeh al-Bukhaari (5705) and Saheeh Muslim (220) it is narrated
that Ibn 'Abbaas (may Allaah be pleased with him) said: The Messenger
of Allaah (peace and blessings of Allaah be upon him) said: "The
nations were shown to me and I saw aProphet with a group of men, a
Prophet with oneor two men, and a Prophet with no one with him. Then a
huge crowd was shown to me,and I thought that they were my ummah, but
it was said to me, 'This is Moosa and his people. But look at the
horizon.' I looked, and there was a huge crowd. Then it was said to
me: 'Look at the other horizon,' and there was (another) huge crowd.
It was said to me: 'This is your ummah, and among them are seventy
thousand who will enterParadise without being called to account or
punished.'"
Then he got up and went into his house, andthe people started
discussing those who would enter Paradise without being called to
account or being punished. Some of them said: Perhaps they are the
ones who accompanied the Messenger of Allaah (S). Some said: Perhaps
they are those who were born in Islam and did not associate anything
with Allaah. And they mentioned several ideas.Then the Messenger of
Allaah (S) came out and said: "What are you discussing?" They told
him, and he said: "They are the ones who did not perform ruqyah or ask
others to do so, and did not believe in omens, and did not use
cautery, and they put their trust in their Lord." 'Ukkaashah ibn
Mihsan stood up and said: Pray to Allaah to make me one of them. He
said: "You will be one of them." Another man stood up and said: Pray
to Allaah to make me one of them. He said: " 'Ukkaashah has beaten you
to it."
The phrase "did not perform ruqyah or ask others to do so" means that
they did not ask others to perform ruqyah for them. Although asking
someone to perform ruqyah is permissible, it is contrary to that which
is better.
The phrase "and did not believe in omens" meansthat they did not
believein omens based on birds or anything else about which people are
superstitious and may give up something that they had decided to do
because of these superstitions. Superstition is haraam and is a form
of minor shirk.
The phrase "and did not use cautery" means that they did not cauterize
with fire to treat sickness, even if it is proven to be of benefit,
because the Prophet (peace and blessings of Allaah be upon him)
disliked it, and because no one can punish with fire but the Lord of
the fire.
The common thread in these three characteristics is that they "put
their trust in their Lord", i.e., they attained the highest degree of
trust (tawakkul) and they did not pay the slightest attention to the
means, and they did not dependon them, rather they depended on their
Lord alone, may He be glorified.
Tawakkul (trust) is the summary of faith, as Sa'eed ibn Habeeb said,
indeed it is the ultimate aim as Wahb ibn Munabbih (may Allaah have
mercy on him) said.
In question no. 4203 youwill find further discussion of this hadeeth,
so please read it. And Allaah knows bestand is most Wise.
Note: Achieving true Tawheed or belief in theOneness of Allaah
cannotbe achieved by mere wishing or pretending, or by empty claims
that have no real essence, rather it is achieved by means of firm
belief thatis rooted in the heart, byachieving true ihsaan that is
confirmed by a good attitude and righteous deeds. The Muslim must
hasten to make the best of every moment of his life and hasten to do
good deedsand acts of worship; he should ignore the hardship and enjoy
the pain, for that which is with Allaah is precious, for that which is
with Allaah is Paradise.
See: al-Qawl al-Sadeed 'ala Maqaasid Kitaab al-Tawheed by Shaykh 'Abd
al-Rahmaan al-Sa'di(may Allaah have mercy on him), p. 20-23.