Shahadah

When a person says the Shahadah and believes in his or her heart and mind , thereby becoming Muslim, their prior good deeds are retained, all of their prior bad deeds are instantly forgiven and they are granted a new beginning.
No god but God - The first part of the Shahadah states that there is no god in existence besides ALLAH, the One, the Only. Those who believe in Him are expected to affirm His Oneness, to worship Him, to love Him, to have hope in His Mercy and Pardon, to fear His anger, and to obey Him. These are ALLAH's rights. Whoever gives ALLAH His rights earns the right to His forgiveness and exemption from His punishment.
Muhammad is the messenger of ALLAH- This means that Muhammad, peace and blessings be upon him, is to be believed in as ALLAH's prophet and messenger. Moreover Muslims are required to equally believe in all of ALLAH's earlier prophets who preceded Muhammad, such as Adam, Noah, Abraham, Moses and Jesus among others. Lastly this also means that Muhammad, peace and blessings be upon him, is ALLAH's final prophet and messenger - foretold by his predecessors, whom no other prophet succeeds.
They surely disbelieve those who say "indeed Allah is one of three" when there is no god save Allah, the One.
Qur'an, al-Maida:73
And do not call upon besides Allah any other god, for there is no god but He. All shall perish save He. His is the Judgement and to Him shall you be brought back.
Qur'an, al-Qasas:88