Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The first obligation upon a person is to affirm the Oneness of Allah Almighty and believe in His Messenger Muhammad ﷺ, by acknowledging the testimony that there is no deity worthy of worship except Allah and that Muhammad is the Messenger of Allah. This is the foundation of Islam upon which all other acts of worship and deeds are built.
Tawhid Was the First Call of All the Prophets
The call of all the prophets began with affirming the Oneness of Allah and worshipping Him alone.
Allah says:
“We never sent before you any messenger without revealing to him: ‘There is no deity worthy of worship except Me, so worship Me.’”
[Al-Anbiya 21:25]
Thus, the foundation of the message of all the prophets was faith in Allah and worshipping Him alone without any partner.
What Did the Prophet ﷺ Command Mu‘adh to Begin With When He Sent Him to Yemen?
When the Prophet ﷺ sent Mu‘adh ibn Jabal (may Allah be pleased with him) to Yemen, where he would meet people from the People of the Book, he explained what he should begin with in calling them to Islam. He said:
“You are going to a people from the People of the Book, so let the first thing you call them to be the testimony that there is no deity worthy of worship except Allah.”
Other narrations of the hadith mention calling them to the Oneness of Allah and belief in the Messenger of Allah ﷺ.
[Agreed upon]
After that, he instructed him to teach them prayer and zakah.
This shows that tawhid and belief in the Messenger ﷺ come before the remaining rulings, because they are the foundation upon which all other acts of worship are built.
What Does “There Is No Deity Worthy of Worship Except Allah” Mean?
It means that Allah alone deserves worship, and no act of worship should be directed to anyone besides Him.
Allah says:
“That is because Allah alone is the Truth, and whatever they invoke besides Him is falsehood.”
[Al-Hajj 22:62]
It is not enough merely to acknowledge that Allah is the Creator. Rather, He must be singled out in worship, obedience, and devotion.
What Does “Muhammad Is the Messenger of Allah” Mean?
It means believing that Muhammad ﷺ is a Messenger sent by Allah to humanity, believing what he informed us of, obeying what he commanded, avoiding what he prohibited, and worshipping Allah according to what he brought.
Allah says:
“Whoever obeys the Messenger has indeed obeyed Allah.”
[An-Nisa 4:80]
Thus, the religion is based on two great foundations: sincerity to Allah and following His Messenger ﷺ.
Why Does Islam Begin With These Two Foundations?
Because an act of worship is accepted when it is devoted sincerely to Allah alone and performed according to the guidance of the Messenger of Allah ﷺ.
The testimony “There is no deity worthy of worship except Allah” establishes sincerity in worshipping Allah, while the testimony “Muhammad is the Messenger of Allah” requires following the Messenger ﷺ.
Then come prayer, zakah, fasting, Hajj, and the rest of the laws of Islam.
Conclusion
The first obligation upon a person is to affirm the Oneness of Allah, believe in His Messenger Muhammad ﷺ, and acknowledge the two testimonies: that there is no deity worthy of worship except Allah and that Muhammad is the Messenger of Allah.
Tawhid is the foundation of the religion, and belief in the Messenger ﷺ requires following what he brought. All acts of worship and obedience are built upon these two foundations.
And Allah knows best.