Why is it not said in the Creed simply, “I believe in God,” rather than with the addition, in one God?
The Creed confesses our faith in “one God the Father” in order to contradict the error of the polytheists (also called pagans, or heathen), who wrongly believe that there are many gods. Some of these religions further teach that their “gods” are not truly the supreme Creator but are themselves subject to some higher power, a confession, despite itself, that such beings are not God at all.
As St Paul, who encountered many polytheists in his ministry, writes: “We know that ‘an idol has no real existence,’ and that ‘there is no God but one.’ For although there may be so-called gods in heaven or on earth, as indeed there are many ‘gods’ and many ‘lords’, yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist” (1Co 8:4–6; see Q238).
In modern times, Mormonism (the religion of the so-called “Latter-Day Saints”) is an example of polytheistic belief presenting itself under a Christian name.
By contrast, Christianity, like Judaism and Islam (although with a major difference), is a monotheistic faith. Yet, the Christian confession of the one God is unique: we confess the one God as the Father who is eternally with his Only-Begotten Son and Holy Spirit. We know the Father only through the Son, “no one who denies the Son has the Father; whoever confesses the Son has the Father also” (1Jo 2:23). The mystery of the Holy Trinity, one in essence and undivided, will be set forth below.