Summary of the concepts of Ahl al-Sunnah
The monotheists among the Christians
A sect among the Christians that says Allah is One, not Three.
They were formerly in Europe, then they emigrated to America.
And today they have entered what is known as the World Church,
which sees the essence of all of Adam's progeny.
And they are currently joining the Protestants in their rejection of the Pope's leadership.
And someone might say,
"Are they not considered Muslims as long as they are monotheists, believing in Allah the Almighty?"
The answer is that Islam is not merely the affirmation that Allah is One.
Rather, this is only one pillar of faith.
And among the remaining pillars is belief in the Messengers,
which necessitates belief in Muhammad, peace and blessings of Allah be upon him,
the Seal of the Prophets and Messengers, sent to all people.
And they do not believe in that.
Therefore, they are disbelievers, as the Prophet, peace be upon him, said:
"By Him in whose hand is the soul of Muhammad, no one from this Ummah hears of me,
be he a Jew or a Christian,
and then dies without believing in what I was sent with,
but he will be among the companions of the Fire."
Narrated by Muslim.
This is aside from the fact that their supposed monotheism is questionable.