There exists some misconceptions towards the importance of doctrines and beliefs in a person’s life, and on the importance of a person’s actions and transactions in his path towards perfection and exaltation and nearness towards Allah (swt).
Which is more important, and on what basis will Man be judged on the Last Day? Are there differing levels of perfection? On what basis are these different levels of perfection distinguished? Is it based on our doctrines and beliefs or on our actions? And what is the Quran’s view on this matter?
The Qur’an focuses on 3 essential doctrines (اصول الدين), which every Muslim must believe in for his actions to have any meaning and value, and differentiates between people based on Monotheism (توحيد), Prophethood (نبوة) and the Return/Day of Judgement (معاد).
These 3 doctrines are the essence of the Islamic Religion (in fact any religion) and cannot be separated from each other. If one wants to simplify religion even further, it can be shown that the Qur’an revolves around one doctrine, and that is monotheism (divine unity). Put even simpler it is the ideology of worshipping non other than Allah (swt). Which is the common denominator amongst all Prophets and Messengers throughout history
So, Monotheism is the source. Prophethood and the Day of Judgement are branches of that source, but remain fundamentally linked. In our ideology (followers of the Holy Household/Progeny), we add Imamate (successorship to Prophethood) and Divine Justice to those branches, which brings the total essential doctrines in Islam to five.
Man is elevated in differing degrees of perfection, and based on the above it is his beliefs and doctrines that give him his elevated status. But is it suffice to assume that belief in religious tenets suffices to elevate man to higher stations of existence? Or is it a combination of correct action and correct doctrines that together allow man to reach such levels of perfection, that he then becomes the prostrated by the Angels?
مَن كَانَ يُرِيدُ الْعِزَّةَ فَلِلَّهِ الْعِزَّةُ جَمِيعًا إِلَيْهِ يَصْعَدُ الْكَلِمُ الطَّيِّبُ وَالْعَمَلُ الصَّالِحُ يَرْفَعُهُ وَالَّذِينَ يَمْكُرُونَ السَّيِّئَاتِ لَهُمْ عَذَابٌ شَدِيدٌ وَمَكْرُ أُوْلَئِكَ هُوَ يَبُورُ
‘Whoso desireth power (should know that) all power belongeth to Allah. Unto Him good words ascend, and the pious deed doth He exalt; but those who plot iniquities, theirs will be an awful doom; and the plotting of such (folk) will come to naught.’[35:10]
So here the ‘good words’ is the correct belief, which is monotheism, and the ‘pious deed’ increases the momentum of that ascent. And what does it mean that ‘good words’ ascend to Him? Where do they ascend to? Isn’t He ‘closer to us than our jugular vein’? Doesn’t He ‘revolve around Man and his heart’? This (physical nearness and remoteness) is a materialistic notion and doesn’t hold any water, since we are talking about immaterial realities. When the Qur’an deals with nearness and remoteness, it is referring to awareness and God-consciousness vis-a’-vis ignorance and not being conscious of the presence of God. (اعبد الله كأنك تراه، فانك ان لم تكن تراه فانه يراك‘ Worship God as if you see Him, and if you don’t (see Him), He surely sees you’).
There is another meaning of nearness to God, and that is in relation to ones level of perfection, wherein the higher the perfection of a believer, the nearer he is, and the opposite is true. And since Allah (swt) is the source of all perfections, theoretically and practically, the more an individual resembles himself to those perfections, the closer he is to the Infinitely Perfect. 
And when the correct beliefs ascend where does this leave the believer? This is where the rational argument comes and states, that when a person believes in a certain doctrine and acts upon it, and he becomes firmly rooted in this belief, then he and his doctrine become one, his actions and his beliefs become intertwined. This is the theory of unity of the intellect (being) and the intelligible (thought) (إتحاد العاقل و المعقول).
A good example that our scholars use to explain this theory further, is the analogy of a piece of coal that is placed in fire for some time and slowly loses all its previous characteristics. In the beginning it is dark and cold, but when left in the fire long enough it becomes red-hot and becomes itself an emitter of fire and a source of heat. In Islamic philosophy this is called  ‘Substantial Motion’ (الحركة الجوهرية).
Therefore, a person’s belief comes at first unsettled and temporary (مستودع), and hence easily shaken by the slightest troubles, and the way to make it settled (مستقر) and part of his existence is with consistent good action. 
 Tafseer Al-Mizan, Allamah Tabataba’i, Vol.17, pg 23.
 See short explanation of theories of Sadr al-Din Shirazi (Mulla Sadra) here.
 Al Qur’an, [6:98]