The Shahadah is the first pillar of Islam. Shahadah is the declaration that there is no god but Allah and Prophet Muhammad, Sall-Allahu alayhi wa sallam, is the Messenger of Allah.
The first part of the Shahadah, that there is no god but Allah attests the Oneness of Allah. There are numerous verses in the Qur'an about Tawheed or the Oneness of Allah.
In the first surah of the Qur'an it says:
"You alone do we worship, and You alone do we ask for help."
Similarly, the Qur'an also states:
"Worship Allah and join no partner with Him." [An-Nisaa 4:36]
The most summarizing expression is in Surah Ikhlas:
"Say, He is Allah, the One. The eternally besought by all. He begetteth not, nor was He begotten. And there is no one comparable to Him." [Al-Ikhlas 112:1-4]
Associating partners with Allah is called Shirk. Shirk is not only the worship of idols, but also offering prayers or supplications to anyone, living or dead, believing that they hold the same attribute as Him. The Qur'an considers Shirk as an unpardonable sin. It says:
"Allah forgives not that partners should be set up with Him; but He forgives anything else to whom He pleases; to set up partners with Allah is to devise a sin most evil indeed." [An-Nisaa 4:48]
"What! Shall I seek for you as god other than Allah, while He has given you superiority over all creations." [Al-An'am 7:14]
The Holy Prophet, Sall-Allahu alayhi wa sallam, is reported to have said:
"Shirk is the greatest of all sins."
The next basic requirement is to believe in all the Prophets of Allah. The Qur'an says:
"He who obeys the messengers obeys Allah." [An-Nisaa 4:80]
Allah sent prophets in different periods of time to communicate His guidance with human beings. They were raised from the nation in which they were sent to preach for. They preached in the languages of there nations as the Qur'an says:
"And We never sent a messenger save with the language of his folk that he might make (the message) clear for them." [Ibrahim 14:4]
"And there not a nation but a warner has passed among them." [Fatir 35:24]
"And for every people there is (sent) a guide." [Ar-Ra'd 13:7]
Prophet Muhammad, Sall-Allahu alayhi wa sallam, is the last Prophet. No prophet shall come after him. He was sent for all mankind as he said:
"Every prophet before me was sent for his own people but I am sent for all mankind."