The teachings of Islam are divided into three basic parts: Islam, Iman, and Ihsan.
The part of Islam discuss the practices to be implemented by a Muslim that is contained in the five following: -
1) Declare Syahadah (testimony) the recognition that there is no god should be worshiped but Allah and Muhammad is the messenger of Allah.
2) Setting up of the five daily prayers in the day and night.
3) Fasting in the month of Ramadan.
4)Paying the zakah to those who are entitled to receive it
5) The pilgrimage to Mecca at least once a lifetime.
The part of iman discuss the beliefs that must be believed by Muslims which contained in the six items as follows: -
1Belief in Allah.
2) Belief in angels.
3) Belief in scriptures of God
4) Belief in the prophets and apostles.
5) Belief in the Day of Judgement.
6) Belief in destiny that all things are determined by God.
The part of ihsan discuss how to become closer to God
which is comprised of two levels: -
1)We believe in Allah as if we see Him
2)If we do not see Him, then be sure that He sees us.
To get the ihsan one must learn the knowledge of how to approximate to God as described by scholars of Sufism in the hundreds volumes of books.
As you know the first pillar of Islam is reciting the testimony of two words: -
1) ٲشهد أن لا ٳله ٳلا ٱلله
Meaning: I bear witness there is no god should be worshiped but Allah
2)وٲشهد أن محمدا رسول ٱلله
Which means: And I bear witness Muhammad is indeed the messenger of Allah.
The first word is a confession of uluhiah (divinity and oneness of Allah). And the second is an affirmation of Nubuwwah (prophethood) and Risalah (the apostolic) of the Prophet Muhammad. These are the two basic knowledge learned in the teaching of Tawheed. Hence it is an obligation for a person who says the Syahadah study the knowledge of Tawheed deeply so it can pull itself from ignorance to knowledge and from suspicion to believe to make his testimony valid before God and beneficial for himself in the world and the hereafter. Saying the Syahadah without knowing the meaning is less beneficial in the world and the hereafter. So we should seriously study the knowledge of Tawheed because it is the kowledge about the meaning of syahadah itself. How do we complete our Islamic duty if we are ignorant of the meaning of the first pillar of Islam.