The understanding of Deen can be dividend in two Components or two branches of Deen:

1. أصـل الــدين / تـوحيــد

The Core or the Principles or Tawheed

2. فـروع الـــدين 

The Branches or the fiqh or the methodology (Shariah)

It is essential that we must distinguish between the Shariah Law of Islam (الشريعة) and the Core (أصـل الــدين).

 A Chart explaining two branches of Deen

Let us first illustrate this with the beautiful examples Allah (swt) gave of good word in following verse:

أَلَمْ تَرَ كَيْفَ ضَرَبَ اللّهُ مَثَلاً كَلِمَةً طَيِّبَةً كَشَجَرةٍ طَيِّبَةٍ أَصْلُهَا ثَابِتٌ وَفَرْعُهَا فِي السَّمَاء

14:24 Have you not considered how Allah presents an example, [making] a good word like a good tree, whose root is firmly fixed and its branches [high] in the sky?

Though the expression literally means "Pure Word," here it stands for "Truthful Saying and Righteous Creed". According to the Qur'an, this 'Saying' and 'Creed" are the acceptance of the doctrine of Tawhid, belief in Prophethood and Revelation, and in the lift of the Hereafter, for it declares these things to be the fundamental truths¹.

Hence, the good word - لا إله إلا الله – which is the essence of our Tawheed can be understood with this example of Good Tree. The core (أَصْلُهَا) of the tree, which is its root are أصـل الدين. These must be firmly ground for this tree to be healthy and to give us good fruit. But if the roots are rotten then what comes out (فَرْعُهَا or branches) of it will also be rotten.

Ali bin Abi Talhah reported that `Abdullah bin `Abbas commented that Allah's statement مَثَلاً كَلِمَةً طَيِّبَةً refers to testifying to لا إله إلا الله (none has the right to be worshipped but Allah) while, كَشَجَرةٍ طَيِّبَةٍ refers to the believer, and that أَصْلُهَا ثَابِتٌ indicates that لا إله إلا الله is firm in the believers' heart, and وَفَرْعُهَا فِى السَّمَآءِ with which the believer's works are ascended to heaven².

A strongly rooted tree is equivalent to strongly rooted Tawheed that can withhold strong winds and storms. It will provide shade and fruits to people that surround it. While a bad word is like bad tree, as Allah (swt) said: وَمَثلُ كَلِمَةٍ خَبِيثَةٍ كَشَجَرَةٍ خَبِيثَةٍ اجْتُثَّتْ مِن فَوْقِ الأَرْضِ مَا لَهَا مِن قَرَارٍ - 14:26 “And the example of a bad word is like a bad tree, uprooted from the surface of the earth, not having any stability”.

"Evil word" is the opposite of "Pure Word" It may be applied to everything that is unreal and wrong but here it stands for any false creed that one might adopt as a basis of one's system of life, irrespective of whether it be atheism or heresy or disbelief or shirk or idol-worship or any other "ism" that has not been brought by a Messenger. And "evil word" (false creed) has no stability because it is against the Law of nature. Therefore, everything in the universe opposes it and refutes it .

Hence, this also illustrates the importance of holding firm on core of our Deen, and building our Tree of Islam on firmly rooted Tawheed. When our Iman is strong and our understanding of Tahweed is strong, it will result in proper understanding of Branches and it will develop stronger practicing Muslims with Taqwa (fear) of Allah in our hearts. Thus, the mankind will benefit from the fruits and بركاة (blessings) of this healthy tree.

Principle of Deen - أصـل الدين

The Core of Islamic System (Deen) is based on understanding of Tawheed (wahdaniyat) of Allah (swt). It is a common message sent by Allah (swt) through thousands of His Messengers, and it has never changed. From Adam (as) , to Noah (as), to Abrahim (as), to Moses (as), to Jesus (as), to Mohammad ((pbuh)) --- this message remained the same --- the Message of Oneness --- the Message of Tawheed. This is considered as Principle of Deen - أصـل الدين 

Thus, Allah (swt) explained that He send us down same guidance through all of His Prophets, i.e. 

• Submit to Allah (swt) alone 
• Denounce and reject Taghoot.

وَلَقَدْ بَعَثْنَا فِي كُلِّ أُمَّةٍ رَّسُولاً أَنِ اعْبُدُواْ اللّهَ وَاجْتَنِبُواْ الطَّاغُوتَ فَمِنْهُم مَّنْ هَدَى اللّهُ وَمِنْهُم مَّنْ حَقَّتْ عَلَيْهِ الضَّلالَةُ فَسِيرُواْ فِي الأَرْضِ فَانظُرُواْ كَيْفَ كَانَ عَاقِبَةُ الْمُكَذِّبِينَ 

16:36 And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): "Worship Allah (Alone), and avoid (or keep away from) Taghoot (all false deities, etc.)." Then of them were some whom Allah guided and of them were some upon whom the straying was justified. So travel through the land and see what was the end of those who denied (the truth). An-Nahl

The core and the root of Deen (Islam) has always been the same amongst all Prophets, even though their Shariats (فروع الدين) were different. That’s why the Prophet (pbuh) has said that all prophets have same Deen and they are like brothers. Hence, they all said that there is no Illah but Allah and they submitted to Allah alone. In every era they prayed according to how Allah (swt) asked them at the time, but the Deen itself remained the same 

The difference of Shariats (between various Prophets: Moses, Jesus, Mohammad etc.) can be understood similar to Naskh (ناسخ) or Mansookh⁴ (منسوخ) in Islamic Shariat. For instance, at one time Muslim prayed facing towards Baitul Muqdas in Palestine, however, it was later changed to Baitul Haram in Makkah. Similarly, Allah (swt) made Saturday for day of worship for Bani Isreal, however, changed it to Friday for Mohammad (pbuh). 

But the core principle of Deen remained same. Allah (swt) never asked to worship to anyone else (or submit to other laws than Allah’s)⁵:   

شَرَعَ لَكُم مِّنَ الدِّينِ مَا وَصَّى بِهِ نُوحًا وَالَّذِي أَوْحَيْنَا إِلَيْكَ وَمَا وَصَّيْنَا بِهِ إِبْرَاهِيمَ وَمُوسَى وَعِيسَى أَنْ أَقِيمُوا الدِّينَ وَلَا تَتَفَرَّقُوا فِيهِ كَبُرَ عَلَى الْمُشْرِكِينَ مَا تَدْعُوهُمْ إِلَيْهِ اللَّهُ يَجْتَبِي إِلَيْهِ مَن يَشَاء وَيَهْدِي إِلَيْهِ مَن يُنِيبُ

42:13 He has ordained for you of religion what He enjoined upon Noah and that which We have revealed to you, [O Muhammad], and what We enjoined upon Abraham and Moses and Jesus – to establish the religion and not be divided therein. Difficult for those who associate others with Allah is that to which you invite them. Allah chooses for Himself whom He wills and guides to Himself whoever turns back [to Him].

The essence of all messages (that came via various Prophets) can be summarized as :

• Submit to Allah as Rabb and Illah
• Reject All Taghoot - other false deities, gods and systems.
• Implement Allah’s Deen (System) and Commands
• Abolish Shirk, Kufr, Fisq and Zhulm.
• The above four will purify the soul – Tazkiya Nafs 

FOOTNOTE

1. Tafhim, al-Mawdudi
2. Tafsir Ibn Khatir.
3. Tafheem, Al-Mawdudi
4. Usool at-Tafseer Bilal Philips

   Means repeal and replacement of Devine Laws. It should be noted that naskh only occurs to divine commands and prohibitions. Naskh cannot occur to statements of fact or Tahweed.

   With respect to Naskh Allah (swt) replied to the doubts created by jews: مَا نَنْسَخْ مِنْ آيَةٍ أَوْ نُنْسِهَا نَأْتِ بِخَيْرٍ مِنْهَا أَوْ مِثْلِهَا - 2:106 We do not abrogate a verse or cause it to be forgotten except that We bring forth [one] better than it or similar to it. This was in response to a doubt which the Jews tried to implant that If both the earlier Scriptures and the Qur'an were revelations from God then why that the injunctions found in the earlier Scriptures had been replaced by new ones in the Qur'an?. But Allah (swt) replied that do you not know that Allah is absolute sovereign and He has ultimate authority to abrogate previous injunctions and cause people to forget them, but He replaces them with something equally or more beneficial.. (Tafheem ul Quran)

   وَإِذَا بَدَّلْنَا آيَةً مَّكَانَ آيَةٍ وَاللّهُ أَعْلَمُ بِمَا يُنَزِّلُ قَالُواْ إِنَّمَا أَنتَ مُفْتَرٍ بَلْ أَكْثَرُهُمْ لاَ يَعْلَمُونَ - 16:101 And when We substitute a verse in place of a verse – and Allah is most knowing of what He sends down – they say, "You, [O Muhammad], are but an inventor [of lies]." But most of them do not know.

   Naskh can be of four types: i) Naskh of Quran by Quran, ii) Naskh of Quran by Sunnah, iii) Naskh of Sunnah by Quran, iv) Nashkh of Sunnah by Sunnah.

   When the divine law is repealed, it was replaced by another law or not replaced at all. e.g.

   Naskh without replacement: Giving charity before talking to Messenger in private: يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا نَاجَيْتُمُ الرَّسُولَ فَقَدِّمُوا بَيْنَ يَدَيْ نَجْوَاكُمْ صَدَقَةً - 58:12 you who have believed, when you [wish to] privately consult the Messenger, present before your consultation a charity”, was repealed by: أَأَشْفَقْتُمْ أَن تُقَدِّمُوا بَيْنَ يَدَيْ نَجْوَاكُمْ صَدَقَاتٍ فَإِذْ لَمْ تَفْعَلُوا وَتَابَ اللَّهُ عَلَيْكُمْ - 58:13 “Have you feared to present before your consultation charities? Then when you do not and Allah has forgiven you,”

   Naskh by an easier law: يَا أَيُّهَا الَّذِينَ آمَنُواْ كُتِبَ عَلَيْكُمُ الصِّيَامُ كَمَا كُتِبَ عَلَى الَّذِينَ مِن قَبْلِكُمْ – 2:183 “O you who have believed, decreed upon you is fasting as it was decreed upon those before you..”. This was replaced by: أُحِلَّ لَكُمْ لَيْلَةَ الصِّيَامِ الرَّفَثُ إِلَى نِسَآئِكُمْ - 2:187 It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations].

   Naskh by a similar law: as in the case of changing direction of Qibla to Masjid ul Haram in Makkah. (Al-Baqara 2:144)

5. Summarized from Iqtida Sirat Mustaqeem – Ibn Taymiya