What is the greatest thing that Allah (SWT) has enjoined?

The greatest thing Allah (SWT) has enjoined is Tawheed (Monotheism).
----------------------------------------------------------------------
What is Tawheed (Islamic Monotheism)?

Tawheed means declaring Allah (SWT) to be the only God who deserves to
be worshipped in truth and confirming all attributes with which He has
qualified Himself or that are attributed to Him by His Last Messenger
Muhammad (SAW).
----------------------------------------------------------------------
What are the aspects of Tawheed?

There are three aspects of Tawheed:

1. Tawheed-ar-Rububiya.

2. Tawheed-al-Uluhiya.

3. Tawheed-al-Asma was-Sifat.
----------------------------------------------------------------------
What is Tawheed-ar-Rububiya?

It is declaring Allah (SWT) to be One and Unique in His work, like
creation, sustenance, bringing to life and causing death, etc.
----------------------------------------------------------------------
What is Tawheed-al-Uluhiya?

It is declaring Allah (SWT) as the Only god to whom all acts of worship
must be dedicated such as Salat (prayers), Zakat (poor tax), Saum
(fasting), Du'a (supplications), Intention, etc.
----------------------------------------------------------------------
What is Tawheed-al-Asma was-Sifat?

It is an affirmation of all the Divine Names and Attributes of Allah
(SWT) in a manner that suits His Majesty, as mentioned in the Quran and
the Sunnah.

How would you describe Ibadah?
It is a comprehensive word comprising deeds and words that Allah (SWT)
loves and is pleased with whether manifested or hidden.
----------------------------------------------------------------------
What are the conditions of Ibadah?
There are two conditions of Ibadah:

1. Sincerity to Allah (SWT).

2. Submission to Allah's Messenger (SAW) (i.e. to act according to his
Sunnah).
----------------------------------------------------------------------
What are some types of Ibadah?
Some types of Ibadah are the prayers (Salat), the obligatory charity
(Zakat), fasting (Saum), the pilgrimage(Hajj), fear (Taqwa) of Allah
(SWT), hope in His Mercy, Seeking His aid, and other acts of worship which
Allah (SWT) has commanded and enjoined.

What is the greatest thing Allah (SWT) has forbidden?

The greatest thing Allah (SWT) has forbidden is Shirk (polytheism).
----------------------------------------------------------------------
What is polytheism?

Polytheism means to believe that there is one who shares Allah (SWT) in
His acts [i.e. ascribing partners or setting up rivals to Allah (SWT)
in His rights].
----------------------------------------------------------------------
What are the types of polytheism?

There are three types of polytheism:

1. The greater polytheism (Shirk Akbar).

2. The lesser polytheism (Shirk Asghar).

3. The inconspicuous polytheism (Shirk Khafi).
----------------------------------------------------------------------
What are the types of greater polytheism (Shirk Akbar)?

There are four types of greater polytheism (Shirk Akbar):

1. The polytheism in invocation [i.e. involving supplications to other
than Allah (SWT)].

2. The polytheism in intentions [i.e. purpose and intentions not for
the sake of Allah (SWT) but directed towards other deities].

3. The polytheism in obedience [i.e. rendering obedience to any
authority against the Order of Allah (SWT)].

4. The polytheism in love [i.e. showing love to others which is due to
Allah (SWT) Alone].
----------------------------------------------------------------------
What is lesser polytheism (Shirk Asghar)?

The lesser polytheism (Shirk Asghar) is Ar-Riya, that means the acts of
worship done to gain praise or fame rather than to please Allah (SWT),
this type of polytheism, however, does not cast the person committing
it out of the fold of Islam.
----------------------------------------------------------------------
What is inconspicuous polytheism (Shirk Khafi)?

The inconspicuous polytheism (Shirk Khafi) implies being dissatisfied
with the conditions ordained by Allah (SWT).
---------------------------------------------------------------------
What is the proof of the inconspicuous polytheism (Shirk Khafi)?

The proof of the inconspicuous polytheism (Shirk Khafi) is the saying
of Prophet Muhammad (SAW):

"The inconspicuous polytheism (Shirk Khafi) is more hidden among this
nation than the track of a black ant over a black stone on a dark
night." (Musnad Ahmad)
----------------------------------------------------------------------
Are good deeds accepted by Allah (SWT) with the polytheism?

Never!!! None of the deeds are accepted when mixed with polytheism.
Allah (SWT) says:

"If they had joined in worship others with Allah all that they used to
do would have been of no benefit to them." (Al-An'am 6:88)

"Verily! Allah forgives not setting up partners in worship with Him,
but He forgives whom He pleases sins other than that." (An-Nisa 4:116)

What are the types of Kufr (disbelief)?
There are two types of Kufr (disbelief):

1. The major Kufr which casts it's people out of Islam.

2. The lessor or minor Kufr which does not cast the one who commits it
out of Islam. It is Kufr of ungratefullness.
----------------------------------------------------------------------
What are the types of major Kufr (disbelief)?

1. The Kufr of denial.

2. The Kufr of arrogance associated with recognition of the truth.

3. The Kufr of doubt.

4. The Kufr of disregard.

5. The Kufr of hypocrisy.
----------------------------------------------------------------------
What are the categories of hypocrisy?
Hypocrisy in Belief is of six types:

1. Denial of the Messenger (SAW).

2. Denial of the thing with which the Messenger (SAW) is sent.

3. Hating the Messenger (SAW).

4. Hating the thing with which the Messenger (SAW) is sent.

5. Rejoicing at the disgrace of Islam.

6. Disliking the supremacy of Islam.
----------------------------------------------------------------------
What is the hypocrisy in deeds and actions?

The hypocrisy in deeds and actions is of five types:

1. When he/she speaks, he/she lies.

2. When he/she promises, he/she breaks it.

3. When he/she is entrusted, he/she betrays.

4. When he/she disputes, he/she acts immorally.

5. When he/she makes an agreement, he/she acts unfaithfully.