Reddit reply in r/muslimtechnet: tell me about islam — October 9, 2022 · irnsecs | Ali Shukri Amin

Islam is a comprehensive Way of life (in Arabic, Din). Islam means "submission," referring to submission to the One True God. This Din is based on six things: foundational principles, worldview, practices, spirituality, a Revealed Book, and the example of the Prophet Muhammad ﷺ (salutations and peace of God upon him).
In brief: there are eight foundational principles (Usul) of Islam. These are the basic tenants of the Din, and no one can be a Muslim (follower of Islam) without ascribing to them - even if they claim to be a Muslim.
Knowledge of Allah, His Messenger, the Din of Islam, and the Qur'an
Al-Bara' (may Allah be pleased with him) narrated:
حَدَّثَنَا مَحْمُودُ بْنُ غَيْلاَنَ، حَدَّثَنَا أَبُو دَاوُدَ، حَدَّثَنَا شُعْبَةُ، أَخْبَرَنِي عَلْقَمَةُ بْنُ مَرْثِدٍ، قَالَ سَمِعْتُ سَعْدَ بْنَ عُبَيْدَةَ، يُحَدِّثُ عَنِ الْبَرَاءِ، عَنِ النَّبِيِّ صلى الله عليه وسلم فِي قَوْلِ اللَّهِ : ( يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ فِي الْحَيَاةِ الدُّنْيَا وَفِي الآخِرَةِ ) قَالَ " فِي الْقَبْرِ إِذَا قِيلَ لَهُ مَنْ رَبُّكَ وَمَا دِينُكَ وَمَنْ نَبِيُّكَ؟ " . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ .
Regarding Allah's saying: (Allah will keep firm those who believe, with the word that stands firm in this world and in the Hereafter (14:27)).
The Prophet (ﷺ) said: "In the grave, when he \[the dead person\] is asked: 'Who is your Lord? What is your religion? And who is your Prophet?'"
\[Jami\` at-Tirmidhi 3120\]
Who Is Allah?
Allah is the Arabic Name of the One True God. Muslims believe in Allah with respect to: creed, sovereign jurisdiction, and gnosticism. Muslim creed ('Aqidah) about Allah is that He is uniquely One (Unicity; Tawhid).
Allah is One in Himself, not being made up of parts or personalities. Allah has no son or father. This type of Oneness is called Wahdaniyyah.