Muslim
Isn't the Trinity a form of polytheism (three gods)?
The Trinity is not three gods but one God in three co-equal, co-eternal Persons — Father, Son, and Holy Spirit — sharing a single, undivided divine nature, a mystery of unity-in-relation rather than a multiplication of deities.
Catholic teaching
Catholics affirm strict monotheism: there is only one God, one divine essence, but that one divine life is eternally lived out in the relations of Father, Son, and Holy Spirit, each fully God, none a separate deity, none created or subordinate in nature.
Biblical foundation
Jesus commands baptism 'in the name' — singular — 'of the Father and of the Son and of the Holy Spirit' (Matt 28:19), and the Shema's monotheism (Deut 6:4) is never abandoned even as the New Testament applies fully divine titles and works to the Son (John 1:1) and the Spirit (Acts 5:3-4).
Scripture references
Historical evidence
The doctrine of the Trinity was carefully defined at the Councils of Nicaea (325) and Constantinople (381) precisely to safeguard monotheism against real polytheistic distortions being proposed at the time, showing the Church has always insisted on one God, not three.
The Church Fathers
“There is one God, in Trinity glorified... we know one God in the Trinity, and the Trinity we believe to have one Godhead.”