The claim and presupposition in this question is that God does the act of which He is capable and if there is an act which He does not have the power to do, He uses certain means to carry them out and achieve His ends. However, since we know that God is All-Powerful, it is unacceptable to say that He carries out His task using any means. Whoever claims that God does an act through the means, his claim is void and totally unacceptable because such a claim implies God's powerlessness.
Such an argument is incorrect and the fallacy used in it is very conspicuous because we see very clearly that many things in the world serve as causes for other things while they themselves are in need of other causes. Thus, they do not have an independent existence of their own. Denying such a clear principle amounts to skepticism, and there is no one to reject the permeation of such a principle in his daily life.
The fact that God, the Almighty, conducts the affairs of the world by way of certain causes does not in any way mean that He Himself is unable to do them. That is because the causes and whatever attached to them do not have any existence of their own. In fact, their existence is pure dependence on God. Therefore, God's actions irrespective of whether they are related to this world or to the unseen matters are conducted through their special causes which on the whole imply God's wisdom not any deficiency in His essence.
As we see in nature, human needs are fulfilled through their natural causes (causes which are not independent in themselves) in the same way as the spiritual matters are conducted through their special causes. We know that some of the divine actions are done through the angels with God's permission. This is not in contradiction with God's power and authority. If no cause were supposed to come into existence, even dependently (dependence in causality), then no being would have existed other than God. That is because it would not have been possible for any being other than God to exist. That is because the existence of every other being is derivative or relative as compared to the real existent.
The universe and whatever is in it are the manifestations and epiphany of God's power. He is Powerful over all things but divine power does not mean that God's actions are done without their special causes. In fact, God, the Sublime, has created the universe on the basis of wisdom and every unseen or seen, visible and invisible, creational and worldly act takes place through its special cause. The angels, for instance, are tasked to conduct many affairs of this world. Also, in the physical world, every natural phenomenon has its special natural cause. For example, if a person feels hungry, he needs food to eat. Although every matter ends up in God and 'there is no strength nor power except through Allah' [i] , the food and water in the natural life is a means for survival. These and all other similar instances do not mean that God is powerless to do an action without an intermediary. In fact, it means that man's material needs are fulfilled through their material causes. Additionally, doing an action through these means does not mean that whoever uses these means has committed a polytheistic act and denied the One God. Indeed, if he considers them as independent causes, he can be regarded a polytheist. The same meaning is applicable to spiritual causes including guidance, conveyance of spiritual graces etc.
Therefore, the principle of causation and the fact that every thing comes into being through its special cause are so clear that they do not require any reasoning. Given this fact, it is an accepted belief in the religion of Ahlul-Bayt (a.s.) that God has not relegated the affairs of this world to their natural causes. There is no doubt that every single cause is a way through which God's will is implemented and materialized. Even those actions which a human being does of his free will take place only when they are willed by God. No one acts independently of God because there is no independent and fully effective cause other than God. Whoever at any level believes in the effectiveness and independence of other causes is a polytheist according to Islamic beliefs.
Based on the foregoing explanation, God's actions are always done through their causes which are not independent and which are under the effect of God's power and will. In this general principle, there is no difference between material causes such as fire which burns and unseen or spiritual causes like the causality of the angels who contrive the affairs of the world and also between volitional actions and non-volitional actions. This is the meaning of complete monotheism (Tawhid) not the nullification or invalidation of the principle of causation and volition.
When it comes to the belief in Tawassol or mediation of an Imam or a perfect human being or a saint – in Shiite religion or in any other religions – what is important to be mentioned in the first place and which should be the basis for this attitude is that the need for an Imam, guide and intermediary does not require self-consciousness towards it. We know that all human beings are naturally inclined to look for a perfect human being so as to symbolize him or to find the truth which he has lost through love of him and tawassol to him. A perfect human being is named "Imam" in Shia religion.
The Holy Quran says:
"یا أَیُّهَا الَّذینَ آمَنُوا اتَّقُوا اللَّهَ وَ ابْتَغُوا إِلَیْهِ الْوَسیلَةَ"
"O you who believe! be careful of (your duty to) Allah and seek means of nearness to Him." [ii]
In reality, this verse guides those believers who truly believe in God and worship him sincerely. The verse implies that piety and nearness to God is easier to achieve through its means and causes.
The next point to mention and that should be kept into account is that Tawassol (invocation) and seeking the help of the Prophet (s) or the Imam (a.s.) never means that they act independently of God. Similarly, none of the causes existing in this universe should be considered as effective independently of God, or else it would be shirk (polytheistic). Generally speaking, there are some Muslims who, as the Quran says, have not attained the status of perfect faith [iii] and a real monotheist. Perhaps, such people may, while relying on material causes, associate a partner for God.
Also, when it comes to spiritual causes such as doing good deeds, availing oneself of a guide and instructor, turning to the Prophet and divine leaders and love of them which is the topic under discussion, it should be said that if these are to be considered as causes, they can be effective to the degree of an individual's belief in the One God and to the amount of his sincerity and love of Him.
Therefore, the principle of monotheism in Islam is an unquestionable principle but the point is that these same causes and our sincere spiritual deeds acquaint us with the reality of monotheism. True belief in the One God is not achievable by mere claim; one cannot be a monotheist and still consider other causes as independent and resort to them with such an attitude and in some other cases criticize to the effect that paying attention to the Imam, prophet and saint is dichotomous with God's power and monotheism. The fact is that, a perfect human being in practice is considered to be a basic element [rukn] of monotheism because a perfect man is one who does not consider himself a real existent; he is the symbol of being a monotheist. In fact, the claim of being a monotheist, if it is to be assessed practically in a divine scale, does not fit anyone other than a perfect human being. Insofar as a Muslim does not detach himself from everything, he is a polytheist (in a relative term though) and his belief in God is imperfect. Therefore, the argument about monotheism (Tawhid) is one thing and its actualization is another. The Quran says:
"وَ ما یُؤْمِنُ أَکْثَرُهُمْ بِاللَّهِ إِلاَّ وَ هُمْ مُشْرِکُونَ" [iv]
"And most of them believe not in Allah without associating (other as partners) with Him!"
For further information in this regard, you can refer to relevant answers under the following titles:
1. Tawassol and Relationship with God without an Intermediary, 542 (site: 590).
2. The Role of Intermediaries in Seeking Nearness to God, 9641 (site: 9681).
3. The Philosophy of Tawassol to the Ahlul-Bayt (a.s), 1321 (site: 1316).