The importance of Tawhid (Monotheism) manifests itself in the following ways:
First: It is the first thing in this religion; it is its goal, end, innermost and apparent essence.
Therefore, it is also the mission of all Messengers and Prophets (peace be upon them).
Second: It is because of Tawhid that Allah created the creation, sent the Messengers and revealed Books. It is also on account of Tawhid that mankind differed and are divided into believers and disbelievers, the fortunate and the unfortunate.
Third: It is the first obligation upon the legally obliged Muslim, the first thing by which man enters Islam and the last thing upon which he should die.
So, the obligatory one is through three things:
1- Purifying it from Shirk that negates the very essence of Tawhid.
2- Purifying it from innovations that negate its mandatory completeness or negate its very foundation if the innovations are the one that turns one to an unbeliever.
3- Purifying it from sins that reduce its rewards and have negative impacts on it.
As for the recommended ones, they are as follows:
1- Actualization of the perfect degree of Ihsan.
2- Actualization of the perfect degree of certainty.
3- Actualization of perfect and beautiful perseverance by not complaining to anyone besides Allah, the Exalted.
4- Actualization of the state of being in perfect contentedness with Allah alone without having a need to any of His creature.
5- Actualization of the degree of reliance on Allah alone by avoiding even some lawful things like making use of lawful incantations and cauterization out of dependence on Allah, the Exalted.
6- Having perfect love for `ibadah (acts of worship) that entails seeking nearness to Allah through performance of many supererogatory deeds.
Whoever actualizes Tawhid as mentioned above and is free from major acts of Shirk; such will have security from perpetual stay in the Hell-Fire.
Whoever is free from the major and minor acts of Shirk and keeps away from major sins, such will have a complete security in this world and the Hereafter. Allah says,
Verily, Allah forgives not that partners should be set up with Him (in worship), but He forgives except that (anything else) to whom He wills. (An-Nisa’ 4:48)
He also says,
It is those who believe and confuse not their belief with injustice (i.e. by worshipping others beside Allah), that shall have security and they are the guided. (Al-An`am 6:82)
On the other hand, the opposite of tawhid is Shirk (polytheism), and it is of three types:
First: Major Shirk that contradicts the very essence of Tawhid. Allah (Glory be to Him) does not forgive it except by repentance. Whoever dies on it will abide in the Fire forever.
Its form is to ascribe a partner to Allah in worship, to call upon that partner as one calls upon Allah(Glory be to Him), to aim for him, to put one’s trust in him, to have hope in him, to love him and to fear him as one loves Allah and fears Him. Almighty Allah says,
Indeed, he who associates others with Allah – Allah has forbidden him Paradise, and his refuge is the Fire. And there are not for the wrongdoers any helpers. (Al-Ma’idah 5-72)
Second: Minor Shirk that contradicts completeness of Tawhid. Minor Shirk is deed that can lead to major Shirk like swearing by others besides Allah and mild showing-off.
Third: The hidden Shirk. This is a kind of Shirk that has to do with intentions and purposes. It can also be major or minor Shirk as explained in the first and the second categories.
Mahmud ibn Labid narrated that the Messenger of Allah (peace be upon him) said, “The thing that I fear for you most is the minor Shirk.” The companions asked: What is the minor Shirk O Messenger of Allah?’ He said, “Showing-off.” (Ahmad)Source Link