This post addresses the most foundational question of all, the one on which every other disagreement between the two faiths ultimately depends: who is God? Islam answers with Tawhid, the absolute and indivisible oneness of Allah. Christianity answers with the Trinity — one God eternally existing as Father, Son, and Holy Spirit. Both traditions insist there is exactly one God. They disagree about what kind of oneness that is, and the disagreement is not a technicality. It is the reason the Qur'an calls the Christian doctrine of God shirk, the one sin it says Allah will not forgive.

What Islam Teaches: Tawhid, the Absolute Oneness of Allah

The single most important doctrine in Islam is Tawhid — the absolute, indivisible oneness of God. Everything else in Islamic theology is subordinate to it, and the greatest sin in Islam, shirk (associating partners with Allah), is precisely the sin of compromising Tawhid. The Qur'an states this with maximum clarity in Surat al-Ikhlas:

"Say, 'He is Allah, [who is] One, Allah, the Eternal Refuge. He neither begets nor is born, Nor is there to Him any equivalent.'" — Qur'an 112:1–4

And explicitly against the Christian Trinity:

"They have certainly disbelieved who say, 'Allah is the third of three.' And there is no god except one God... The Messiah, son of Mary, was not but a messenger; [other] messengers have passed on before him. And his mother was a supporter of truth. They both used to eat food. Look how We make clear to them the signs; then look how they are deluded." — Qur'an 5:73, 75

Allah, in Islamic theology, is a single, undivided, uncomposed Person. He has no partners, no equal, no offspring, and no plurality of any kind within His being. He is described as all-seeing and present everywhere in His knowledge and power (Qur'an 40:20; 2:115), the sole Creator and Sustainer of everything (Qur'an 3:191), all-knowing and all-powerful (Qur'an 2:268; 6:61–62). Classical Islamic theologians such as al-Ghazali (d. 1111) systematized these into the ninety-nine names/attributes of Allah, but always within a framework that treats any plurality of persons within the divine essence as the unforgivable sin of shirk — the one sin the Qur'an says Allah will not forgive if a person dies in it: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills" (Qur'an 4:48; cf. 4:116).

It should be noted fairly that Islamic Tawhid is not a rejection of a caricatured "three gods" — Muslim theologians have historically understood, and rightly rejected, tritheism (three separate gods). Where Islamic theology and Christian theology actually part ways is over whether the one true God can exist as a plurality of persons within a single, undivided essence — precisely what the historic Christian doctrine of the Trinity affirms and what Tawhid, as classically defined, forecloses by definition.

What Christianity Teaches: One God, Three Persons

Christianity is emphatically monotheistic. The Bible's own foundational confession of faith, the Shema, is as absolute a statement of divine oneness as anything in the Qur'an:

"Hear, O Israel! The LORD is our God, the LORD is one!" — Deuteronomy 6:4

The Hebrew word translated "one" here is אֶחָד (ʾeḥād; BDB 25; HALOT 29) — the ordinary Hebrew word for "one," but notably a word that elsewhere describes a composite unity, as when a husband and wife "shall become one flesh" (Genesis 2:24). This is distinct from יָחִיד (yāḥîd, "solitary, only one"), the word Hebrew uses when it wants to stress a strictly singular, undifferentiated unit. The Shema excludes polytheism and the worship of any being besides YHWH; it does not by its grammar exclude a unity of persons within the one Being of God. As the existing study on the Trinity observes, "the numerical oneness of God affirmed here is the oneness the Trinity maintains: one being, one nature, one God — in three persons".

The historic Christian doctrine of the Trinity holds three affirmations simultaneously, none of which may be surrendered without distorting the biblical witness: there is one God (Deuteronomy 6:4; Isaiah 44:6; 1 Corinthians 8:4–6; 1 Timothy 2:5); the Father, the Son, and the Holy Spirit are each fully God (John 1:1; Acts 5:3–4; Matthew 3:17); and the three persons are genuinely, personally distinct from one another (John 1:1–2; John 14:16; John 17:1). Old Testament hints of this plurality-within-unity appear at the very first verse of Scripture — "the Spirit of God was hovering over the surface of the waters" (Genesis 1:2) as a distinct divine agent alongside "God," and the divine self-address "Let Us make man in Our image" (Genesis 1:26) — and reach a striking clarity in Isaiah 48:16, where the divine Speaker (identified with YHWH's own creative authority in the preceding verses) declares, "the Lord GOD has sent Me, and His Spirit" (Isaiah 48:16), naming three distinct agents — the Sender, the Sent One, and the Spirit — within a single divine utterance. John Oswalt, commenting non-Calvinistically in the NICOT series, calls this "perhaps the most explicit anticipation of Trinitarian theology in the OT prophets" ^[1].

The New Testament brings this plurality into full, explicit view. At Jesus' baptism, all three Persons are simultaneously present and personally distinct: the Son stands in the Jordan, the Spirit visibly descends as a dove, and the Father speaks audibly from heaven (Matthew 3:16–17) — a scene R.T. France calls "the most vivid Trinitarian scene in the Gospels... a revelation of the divine being in three persons that no other moment in the Gospels matches" ^[2]. The Great Commission commands baptism "in the name [singular] of the Father and the Son and the Holy Spirit" (Matthew 28:19) — one shared divine name for three distinct Persons. Paul's apostolic benediction names all three as distinct sources of grace, love, and fellowship in a single sentence (2 Corinthians 13:14).

Christianity's monotheism is therefore not the same kind of monotheism as Tawhid. Both traditions agree there is exactly one God. They disagree, fundamentally, on whether that one divine Being subsists as an undifferentiated singularity or as an eternal communion of three co-equal, co-eternal Persons. That disagreement is not a peripheral detail; it is the reason the Qur'an calls the Christian doctrine of God shirk, the unforgivable sin, and the reason historic Christian creeds anathematize any denial of the full and equal deity of Father, Son, and Spirit.

Where the Two Descriptions of God Diverge Further

Beyond the question of plurality-within-unity, the Bible and the Qur'an/Hadith describe God's nature differently on several further points worth noting honestly, since these differences are not always visible from the shared vocabulary of "one," "all-knowing," and "all-powerful" that both traditions use.

Knowledge of the future. The Bible presents YHWH's knowledge as total and settled from eternity: "I make known the end from the beginning, from ancient times, what is still to come... My purpose will stand, and I will do all that I please" (Isaiah 46:9–11). The Qur'an, by contrast, repeatedly uses language suggesting Allah tests people to find out things about them: "that Allah may know those who believe" (Qur'an 3:140–142) and "that He might know the true believers... and that He might know the hypocrites" (Qur'an 3:166–167). Classical commentators such as Ibn Kathir and al-Tabari typically resolve this by reading "know" as "make manifest" or "cause to be known" rather than as genuine acquisition of new knowledge by Allah, which is the standard Sunni theological position safeguarding divine omniscience; the wording itself, however, is worth noting fairly as a point where the plain text of each scripture reads differently even after each tradition's own doctrinal resolution.

Visibility and corporeality. The Bible describes God as spirit — "God is spirit, and His worshipers must worship in spirit and truth" (John 4:24) — who is nonetheless able to appear and act within creation, as when three visitors, one of them addressed as "the LORD," ate a meal with Abraham at Mamre (Genesis 18:1–8). Sunni Hadith literature, following a strand of Sifat (attributes) theology associated especially with traditionalist (Athari) scholarship, affirms that Allah has a literal Face (Qur'an 55:27), Eyes (Qur'an 52:48), Hands (Qur'an 39:67), and a Shin that will be uncovered on the Day of Resurrection so believers may prostrate before it (Sahih al-Bukhari 7439, on the Shin being uncovered on the Day of Resurrection) — affirmed by traditionalist scholars bila kayfa ("without asking how," i.e., without specifying the manner) as real divine attributes unlike any creaturely attribute, while other Islamic theological schools (notably Ash'ari and Mu'tazili theologians) interpret such verses metaphorically rather than literally. This is a genuine point of internal Islamic theological debate, and it is presented here descriptively.

Self-sufficiency. The Bible presents God as wholly self-sufficient, needing nothing from His creation: "nor is He served by human hands, as though He needed anything" (Acts 17:25). A well-known hadith reports Muhammad teaching that if human beings ceased to sin, Allah would remove them and create a sinning people instead, "for He loves to forgive" (Sahih Muslim 2749) — a saying meant to illustrate the vastness of divine mercy, but one that raises, from a Christian vantage point, the question of whether God's expression of a core attribute (mercy) is here made contingent on the existence of sinners He must forgive.

Despite Muslims believing that they sincerely worship the Creator of Abraham, Moses, and Jesus — it is clear they worship a different deity. The point of comparison is narrower and more precise: the God described in the Qur'an and Hadith and the God described in the Old and New Testaments are not, on careful textual comparison, interchangeable descriptions of a single, identically conceived deity. They differ in essential nature (unitarian singularity versus triune plurality), in the extent and mode of divine self-disclosure (Allah remains fundamentally transcendent and unknowable in His essence; the God of the Bible discloses His own nature by becoming flesh — the subject of The Jesus Islam Cannot Accept), and, as shown above, in some specific textual claims about God's knowledge, form, and self-sufficiency.