I bear witness there is no God but ALLAH and I bear witness that Muhammad is HIS Messenger.
This is the oath that brings a person into the fold of Islam and unites the person with the over 1.4 billion Muslims in the world today. It is a simple phrase, with enormous meaning.
First, one must testify, bear witness, take the oath, and believe, that there is only one God, whose proper name is ALLAH, Subhanahu wa ta'ala. ALLAH, Subhanahu wa ta'ala, is the Creator, the First and the Last, the All-Seeing, the All-Knowing, the All-Encompassing being that created the Heavens and the Earth, the Just, the Most Gracious, the Most Merciful, the Sovereign, the Holy One, the Source of Peace, the Guardian of Faith, the Preserver of Safety, the Exalted in Might, the Irresistible, the Supreme, the Evolver, the Bestower of Forms, the Forgiving.
ALLAH, Subhanahu wa ta'ala, has no partners and ascribing partners to HIM is the only sin which will not be forgiven.
Next, one must testify, bear witness, take the oath and believe that Prophet Muhammad, salla ALLAHu alaihi wa sallam, is the Messenger of ALLAH, Subhanahu wa ta'ala, sent as a mercy for all creatures. Muslims believe in all the Prophets and Messengers, alaihim assalaam. That includes the ones whose names have been revealed (as listed above) as well as the over 124,000 others whose names have not been revealed. Muhammad, salla ALLAHu alaihi wa sallam, is the seal of the Prophets and Messengers, alaihim assalaam. There will be no more Prophets sent to mankind, as the final Message has been delivered in the form of the Quran.
To be a Muslim, one must utter this simple phrase, believe in it, and hold true to its meaning. That brings one into the fold of Islam. This is the most important pillar of Islam. The belief in One God, ALLAH, Subhanahu wa ta'ala, is referred to as Tawheed and is the basis of Islamic belief. All acts of worship are based on this.