Shahadah of Faith
The first pillar of Islam is the Shahadah of the faith. It is essential for every Muslim to bear witness of faith that is "there is no god but ALLAH and Muhammad (P.B.U.H.) is his prophet". It is not only essentials for the Muslims only rather if any non-Muslim enters Islam, he/she also has to bear witness of the faith as well. Believing in the oneness of God (ALLAH) is the basic concept of Islam.
There are two parts to this faith.
The First Part is "There Is No God but ALLAH"
In this part, the faith and absolute authority of ALLAH is defined. He is the only to whom we worship. No one can be equal to Him. It is also described in the Holy Quran as well. In Surah Ikhlas it is said that Say, He is ALLAH, the One (1). ALLAH, the eternal, Absolute (2). He neither begets nor is born (3). Nor is there to Him any equivalent (4).
These verses of the holy Quran truly express the oneness of the ALLAH, almighty. It is also described in very clear words that no one can be equivalent to him. The powers He has are supreme.
He is a Master of the day of judgment.
The Second Part is "Muhammad (P.B.U.H.) is His Prophet"
In this part, the Prophethood of Muhammed is defined that Muhammad (Peace be upon him) is the messenger of the ALLAH, almighty.
There were about 124,000 messengers sent by the ALLAH almighty to call mankind towards ALLAH and to follow the directions given by ALLAH. All of these messengers tried their level best according to the orders of ALLAH. Muhammad (Peace be upon him) was the last messenger and prophet sent by the ALLAH for the guidance and success of mankind.
This concept is also explained in the holy Quran in detail. In Surah Ahzab, it is said that "Muhammad is not the father of any of your men, but he is the messenger of ALLAH and the Seal of the Prophets and ALLAH has full knowledge of all the things".
Now the teachings of Muhammad (Peace be upon him) are being followed and will be followed till the day of judgment. No one will come as a messenger of the ALLAH after Muhammad (Peace be upon him).
No one can be considered as Muslim until he accepts the faith described above and witnesses it. Whenever any non-Muslim accepts Islam, he will have to bear witness of that faith as it is the basic step to enter the religion of Islam.