Now that we have gone through the axiom of monotheism and accepted it as a logical and rational principle, we should note that this belief is much more complicated than it seems. In order to reach a firm belief in the existence of God and His Oneness, one has to follow each of its stages.

These degrees include “unity of the essence of Allah”, “unity of the attributes of Allah”, “unity in what Allah does” and “unity in worship”. The first three of these belong to the realm of theoretical monotheism, while the last one is among practical monotheism. Here we will have a brief look at each of these steps.

1. Unity of the Essence of Allah

This stage of monotheism is defined as believing that God is One and Unique in His essence. The very first thing that one assumes regarding Allah is that of His Self-dependence; He is a being who is not dependent on any other beings in any way, while everything depends on Him and seeks His help.

Secondly, one has to believe in His Creatorship; that He is the creator and the ultimate source of all the existing things, all things are from Him, and He is not from anything. Accordingly, He is called the First Cause. The unity of essence, thus, means that this being is not multiplicable, and there is nothing like it. Since the self-existing truth is one, then this world has only one source and one end; it has neither originated from various sources nor will return to various ones, the whole universe has one center, one pole, and orbit.

2. Unity of the Essence and Attributes of Allah

It is defined as the recognition that the essence and the attributes of Allah are identical and that His various attributes are not separate from each other. While the Unity, in essence, refers to God’s Oneness, denying the existence of any peer or like for Him, the Unity in attributes means the negation of any kind of multiplicity or plurality within His essence.

All the attributes implying perfection and beauty belong to Him, and they are not at all separate from Him. The separation of the essence from the attributes and their separation from each other are the characteristics of a limited being, while God is infinite and such characteristics cannot be ascribed to Him. Accordingly, Allah has attributes which are unlimited – like His own self – and identical with His essence.

3. Unity in What Allah Does

The Unity of His work is to recognize that the world with all its systems is the work of Allah alone and has originated from His will only. Nothing in this world has occurred on its account and independent from God; every agent, cause or power owes its existence and effect to Him.

As He has no partner in His essence, He has no peer in what He does either. While human beings, as one of these agents and causes, have control over their own actions and can influence their destiny, they are not at all out of God’s will and supervision; otherwise, we should consider them as God’s partners, which will ultimately deny God’s unity in essence.   

4. Unity in Worship

Reaching this stage is when one only worships God and has prepared himself for praying Allah, the One. Based on Islamic doctrine, the act of worship has its own degrees, the clearest of which is the performance of the rites for Allah’s glorification and exaltation.

According to Holy Quran, worship is not limited to the actual performance of prayer but includes any form of spiritual orientation and idealization; the person whose purpose and motivation in life only satisfies his own whim has, in fact, led his spiritual direction toward it and thus worshiped it instead of God. So, the unity in worship is to see Allah as the only one who is fit to be worshiped and obeyed unconditionally and regard Him as the only purpose and direction of one’s conduct.

What Is the Difference between Theoretical and Practical Monotheism?

The first three degrees are all theoretical and a matter of creed - they should be recognized and acknowledged, and require rational thinking- while the fourth one is practical and a necessity for all of our actions.

Moreover, theoretical monotheism gives an insight to perfection while practical monotheism is the actual movement toward it; it is only seeing God’s Oneness and finding knowledge about it while practical monotheism is experiencing and feeling these beliefs tangibly in all walks of life.

In other words, theoretical monotheism provides the basis and foundation of practical monotheism; on the other hand, without bringing those beliefs into action, they would become deficient and imperfect.

Have you ever thought about which among the thousands of blessings Allah granted us is the most significant? It is not an easy task, but if you carefully think about it, you may guess the answer. 

As our dear Prophet (PBUH&HP) said (1), the greatest blessing is intellect. Through this remarkable gift, humans have been recognized as the pinnacle of all creations, even angels bow ed down to him expect for Satan. Let us even think about this matter rationally, setting aside religious teachings . Throughout history, the inherent power of humans against wild animals has not primarily been their physical strength or survival capabilities in harsh natural conditions.

Instead, it's their intellect that empowers them to create weapons and shelter, making them the most resilient species on Earth.

Our great Prophet and dear Imams (PBU Them) have consistently mentioned that intellect is one of humanity’s paramount gifts that must be integrated into our lives. Even the Holy Quran has numerous verses about intellect and wisdom, beautifully highlighting their profound importance. If you read the Holy Quran frequently, you have likely seen the words like “They are reasoned” or “They are contemplating” in many verses. These instances underscore how dearly  Islam values intellect and stresses its role in our lives. Let’s now explore some verses here that show us the significance of intellect:

[This is] a blessed Book that We have sent down to you, so that they may contemplate its signs, and that those who possess intellect may take admonition.   
(The Holy Quran 38:29)

They ask you concerning wine and gambling. Say, ‘There is a great sin in both of them, and some profits for the people, but their sinfulness outweighs their profit.’ And they ask you as to what they should spend. Say, ‘All that is surplus.’ Thus does Allah clarify His signs for you so that you may reflect
(The Holy Quran 2:219) 

Indeed, We have sent it down as an Arabic Quran so that you might understand . (The Holy Quran 12:2)

Is he who supplicates in the watches of the night, prostrating and standing, apprehensive of the Hereafter and expecting the mercy of his Lord...? Say, ‘Are those who know equal to those who do not know?’ Only those who possess intellect take admonition.
(The Holy Quran 39:9)

The Prophet (PBUH&HP) stated “The good of this world and the hereafter lies in knowledge, and the evil of this world and the hereafter lies in ignorance.” (2)

Imam Sadigh (AS) said “I wish to see none of you [Shias] but in two categories: a scholar or a student.” (3)

Imam Ali (AS)proclaimed “Indeed, knowledge is the lifeblood of hearts, enlightening blinded eyes and empowering incapacitated bodies.” (4)

References: 

1.  Allah (SW) has not created anything better for man than reason .
2.   Bihar al-Anwar (79th Volume – page 170)
3.   Al-Amali Tousi (Page 303)
4.   Tuhaf al-Uqul (page 28)