Both monotheism and polytheism have degrees and stages. Unless a man passes through all the stages of monotheism, he cannot be a true monotheist.
To acknowledge the unity of His essence means that Allah is One in His essence. The first impression of Allah which anybody has is that of His Self-dependence. He is a Being who is not dependent on any other being in any way. In the words of the Holy Qur'an, He is Ghani (the Absolute). Everything depends on Him and seeks His help. He is independent of everything. The Holy Qur'an says:
"O men, it is you who stand in need of Allah. As for Allah, He is above all need, worthy of praise." Sermon - 1, p. 137, Peak of Eloquence, ISP, 1984)
In the first passage it has been affirmed that Allah has attributes (whose attributes are not limited by any limitation). In the second passage also it is confirmed that He has attributes, but instruction has been given to impute no attributes to Him. The wording of these passages shows that the attributes which He has are unlimited like the limitlessness of His own self, that they are identical with His essence, and the attributes from which He is free are those which are limited and separate from His essence and from each other. Thus the unity of the Divine tributes means to acknowledge the unity of Allah's essence and His attributes.
Unity of His work means to recognize that the world with all its systems, ways, causes and effects is the work of Allah alone and has originated from His will. Nothing in the world is self-existing. Everything depends on Him. In the words of the Holy Qur'an, He is the sustainer of the whole world. The existing things are not independent with regard to their effect and causation. As a result, as Allah has no partner in his essence, similarly He has no partner in His work. Every agent and every cause owes its existence and effectiveness to Him and depends on Him. All power as well as ability to do things belong to Him alone.
Man is one of the existing things and hence a creation of Allah. Like other things he is effective as far as his own work is concerned and unlike of them, he is the master of his own destiny. But Allah has in no way delegated His powers to him. Thus he has not got complete discretion.
"By the power of Allah I stand and sit".
The belief that any being, whether man or a being other than man, has a complete discretion, amounts to believing that being to be a partner of Allah as far as independence of activity is concerned. As independence in activity amounts to independence in essence it is contrary to Allah's unity of essence, what to say of His unity of work.
"All praise is due to Allah who has taken no spouse nor a child. He has no partner in His sovereignty, nor has He a helper to help Him out of weakness. Therefore glorify Him a great deal."