Muslim
Isn't the Trinity shirk (polytheism) against tawhid?
The Trinity is not a claim about three gods but about one God who exists as three co-equal, co-eternal Persons sharing a single undivided divine nature; Christians affirm the absolute oneness of God's essence as firmly as Islam does, differing only on how that unity is internally constituted.
Catholic teaching
The Church solemnly teaches, against every accusation of tritheism, that there is only one God, and that Father, Son, and Holy Spirit are not three gods nor three parts of God but the one same undivided divine substance really distinguished only by their relations of origin.
Biblical foundation
The Shema, 'Hear, O Israel, the LORD our God, the LORD is one' (Deut 6:4), remains fully affirmed by Christians, who read the New Testament's threefold naming of Father, Son, and Spirit (Matt 28:19; 2 Cor 13:14) not as adding gods but as revealing the inner life of the one same God confessed in the Shema.
Scripture references
Historical evidence
The Council of Nicaea (325) and Constantinople (381), centuries before Islam, defined the Trinity precisely to exclude tritheism, condemning any suggestion of three separate gods as heresy — showing the doctrine was formulated to safeguard, not undermine, strict monotheism.
The Church Fathers
“No sooner do I conceive of the One than I am illumined by the splendor of the Three; no sooner do I distinguish Them than I am carried back to the One.”