All the prophets of God including; Adam, Noah, Abraham, Isaac, Jacob, and the tribes, Jesus, and Muḥammad (peace be upon them), invited the people to the same thing, to worship The One God without any partners, son or associates.
Allāh, The Exalted said: ” Say you (0 Muḥammad ): “This is my way, I invite unto Allāh (i.e. to the Oneness of Allāh – Islamic Monotheism) (with sure know/edge), land whosoever follows me (also must invite others to Al/ah i.e. the Oneness of Allāh – Islamic Monotheism) with sure knowledge. And Glorified and Exalted is Allah (above all they associate as partners with Him), and I am not of the polytheists. ” (Al Qurʾān, 12:108)
Ibn ʿAbbās (a companion of the Messenger of Allāh [ṣallallāhu ʿalayhi wa-sallam.)) narrated: When Allāhs’ Messenger sent Muʿadh to Yemen, he said, “You will come upon the people of the book (Christians and Jews), let your first act is to call them to testify’ that – There is nothing worthy of worship in truth but Allāh”. (Al Bukharī #1389)
All the prophets of God including; Adam, Noah, Abraham, Isaac, Jacob, and the tribes, Jesus, and Muḥammad (peace be upon them), invited the people to the same thing, to worship The One God without any partners, son or associates. In order to understand why we must worship Allāh (The One God), we must first accept the Oneness of the Lordship of Allāh i.e. Thuhid-ar-Rubiyah. This means to believe that there is only One Lord for everything that exists, its Creator, Organizer, Originator, Planner, Sustainer, Authority, Giver of Security, etc. and this is Allāh (One God) The belief in the Lordship of One God cannot be separated by the act of worshipping the One God i.e. Tawḥīd-al-Uluhiyah. This means to believe that none has right to be worshipped (e.g. praying, invoking, asking for help from the unseen, swearing, sacrificing, pilgrimage, fasting, giving charity, etc.) but Allāh The final aspect of Tawḥīd is the Oneness of the Names and Attributes of Allāh. This means:
1) We must not name or attribute anything to Allāh except with what Allāh or His Messenger has named or attributed to Him.
2) None can be named or attributed with the Names or Attributes of Allāh. e.g. The Mighty (al-Aziz)
3) We must confirm all of Allāhs’ Names and Attributes stated in His book (The Qurʾān) or mentioned through His Messenger Muḥammad without changing them or ignoring them completely or twisting the meanings or giving them a resemblance to any of the created things These three aspects of Tawḥīd are included in the meaning of La ilāha illā-Allāh (None has the right to be worshipped except Allāh) It is essential to follow Allāhs’ Messenger, Muḥammad (ṣallallāhu ʿalayhi wa-sallam) and it is part of the worship of the One God (Tawḥīd al-Uluhiyah). This is included in the meaning of” I testify that Muḥammad (ṣallallāhu ʿalayhi wa-sallam) is the Messenger of Allāh,” and this means, “None has the right to be followed after Allāhs’ Book (The Qurʾān), but Allāhs’ Messenger,” Allāh said:”And whatsoever the Messenger gives you, take it, and whatsoever he forbids you, abstain from it”. (Qurʾān 59:7)
La ilāha illā Allāh is the foundation of Tawḥīd (Islamic Monotheism) and Islām. It is a complete system of life, through which, all forms of worship (of Allāh) are realized. That happens when a Muslim submits himself to Allāh, and calls upon Him alone, and refers all issues (of right and wrong) to His Law, to the exclusion of all other systems of law
“The word ilah (i.e. God), means the One who is obeyed and not defied, out of one’s sense of awe and reverence, love, fear, and hope, placing one’s trust in Him, asking Him and supplicating to Him alone. Whoever directs any of these matters (which are the rights of Allāh) to a created being, has detracted from the sincerity of his statement La ilāha illā Allāh. And he has worshipped that created being to the extent that he directed those matters towards it”
The Prophet (ṣallallāhu ʿalayhi wa-sallam) said: ” Whoever says La ilāha illā Allāh sincerely will enter Paradise.” (Reported by Bazzār and declared authentic by Al-Albani in Ṣaḥiḥ Al-Jami)
The sincere person is the one who understands, acts according to its requirements, invites others to it, and gives it precedence over all other issues, because it is the concise formula of Tawḥīd (Islamic Monotheism) for which human beings are created.
Therefore Lā ilāha illā Allāh will benefit the one who says it if he conforms to its meaning in his life, and doesn’t nullify’ it by associating partners with Allāh, such as supplicating to the dead or calling upon the living who are absent The Prophet (ṣallallāhu ʿalayhi wa-salām) said “Whoever says La ilāha illah, it will be his salvation someday, no matter what befalls him before that”(This was reported by Baihaqi, declared authentic by Al-Albani in Aḥadīth Ṣaḥiḥah-#l932)
In conclusion, it is from this understanding of Tawḥīd [Islamic Monotheism] that confirms what Muslims already know that Islām is the only Monotheistic Religion and way of life acceptable to the Creator.