Tawhid and the kalimah
The foundation of Islam is tawhid — knowing and affirming Allah as One in His lordship (creating and sustaining), in His right to be worshipped, and in His names and attributes.
The kalimah
لَا إِلٰهَ إِلَّا اللّٰهُ مُحَمَّدٌ رَسُولُ اللّٰهِLa ilaha illallahu Muhammadur-RasulullahThere is no god but Allah; Muhammad is the Messenger of Allah.
The testimony (shahadah)
أَشْهَدُ أَنْ لَا إِلٰهَ إِلَّا اللّٰهُ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُAshhadu an la ilaha illallahu wahdahu la sharika lah, wa ashhadu anna Muhammadan ‘abduhu wa rasuluhI bear witness that there is no god but Allah alone, without partner, and that Muhammad is His servant and Messenger.
Tawhid in brief — Surah Al-Ikhlas
بِسْمِ ٱللَّهِ ٱلرَّحْمَٰنِ ٱلرَّحِيمِقُلْ هُوَ ٱللَّهُ أَحَدٌ ١ ٱللَّهُ ٱلصَّمَدُ ٢ لَمْ يَلِدْ وَلَمْ يُولَدْ ٣ وَلَمْ يَكُن لَّهُۥ كُفُوًا أَحَدٌۢ ٤Qul huwal laahu ahad Allah hus-samad Lam yalid wa lam yoolad Wa lam yakul-lahu kufuwan ahad(1) Say, "He is Allah, [who is] One, (2) Allah, the Eternal Refuge. (3) He neither begets nor is born, (4) Nor is there to Him any equivalent."Surah Al-Ikhlas 112:1-4
Shirk — the greatest sin
Shirk is associating partners with Allah in His worship or His unique attributes — such as asking anyone besides Allah for children, provision or healing, prostrating to graves or idols, or believing amulets benefit by themselves.
إِنَّ ٱللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِۦ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُ ۚ وَمَن يُشْرِكْ بِٱللَّهِ فَقَدِ ٱفْتَرَىٰٓ إِثْمًا عَظِيمًا ٤٨Innal laaha laa yaghfiru ai yushraka bihee wa yaghfiru maa doona zaalika limai yashaaa'; wa mai yushrik billaahi faqadif taraaa isman 'azeemaaIndeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly fabricated a tremendous sin.Surah An-Nisa 4:48
Whoever dies knowing that there is no god but Allah will enter Paradise. (Muslim 26)