Here’s a lovely article on why the pillars of Iman are arranged as they are:
The Wisdom Behind the Arrangement of the Pillars of Iman
Shaikh Saalih Al-Fawzan
There is wisdom behind – and Allaah knows best- the arrangement of the pillars of Eemaan in the verses of the Qur’aan and the ahaadeeth, although the waaw (و) doesn’t exactly necessitate arrangement:
The pillars of Eemaan have begun with belief in Allaah because it is absolutely the foundation and (all) the other pillars aside from it come in succession to it.
Then comes the mention of belief in the Angels and Messengers because they are the mediators between Allaah and His creation regarding the conveyance of His messages. Thus, the Angels descend with the revelation (from Allaah) and bring it to the Messengers and the Messengers convey it to the people. The Most High says,
Thereafter is mention of belief in the books because they are the criterion and returning point that which the Prophets and Messengers, ‘alayhim as-salaatu was-salaam, came with by way of the Angels from Allaah the Most High, in order to judge between the people in that which they differ. The Most High says,
Afterwards is mention of belief in the Last Day as it is the appointment of recompense for (all of) the actions which is the outcome and fruits behind belief in Allah, His Angels, His books and His Messengers or the denial of them. For that reason, from the adequacy of the Divine Justice (of Allah) is the establishment of this Day to judge between the oppressor and the oppressed and the establishment of ‘adl (equity) between the people.
Lastly is the mention of belief in the Qadaa’ and Qadr due to its extreme importance in driving the believer towards the performance of righteous actions and adopting the proper and beneficial means (i.e. in obtaining provisions, etc.) together with reliance upon Allaah, the One free from all imperfections. In order to make evident that there isn’t any contradiction between the legislation of Allaah that He sent His Messengers with and revealed His books based upon and between His judgement and pre-decree. Contrary to those who erroneously believe that from the innovators and polytheists who say,
“…If Allah had willed neither we nor our fathers would have worshipped aught but Him, nor would we have forbidden anything without (a command from) Him.”
They (tried) to justify that which they were upon from disbelief (claiming) that Allaah decreed it for them. So if He decreed it for them then He must have been pleased with it from them – as they claim. So Allaah rebutted them with the fact that if He was – so called – pleased with it then He would not have sent His Messengers with the disavowal of it, as He says,
“…Then! Are the Messengers charged with anything but to convey clearly the Message?”