Shaykh al-Islām Ibn Taymiyyah (d. 728 AH) said:

“The Christians say: ‘God is One, the Creator is One, the Sustainer is One.’

This [phrase] is true in and of itself except that the Christians contradict this in the words of their own creed: ‘We believe in one God; Jesus the Messiah, the only son of God. The true God from the true God. Made from the essence of his Father, equal to Him in essence.’

Here, the Christians have confirmed the existence of two gods followed by the Holy Spirit as a third by claiming that they prostrate before him. Thus, they confirm three [different] gods.

They then say: ‘We confirm but one god.’

This is a brazen contradiction and an attempt to unite between two opposites. [An attempt to reconcile] negation and confirmation [together].

It is for this reason that many scholars postulate that, in general, it is possible to comprehend the opinions of mankind except the opinions taken by the Christians. This is because the ones who originally put forward these ideas did not fully understand what they themselves were suggesting. Rather, they spoke with ignorance and tried to bring together polar opposites within a single belief. In fact, some of these scholars have pointed out that if we were to bring together a group of ten Christians, they would espouse eleven different opinions. Other [scholars] have even said that if you were to ask a Christian [man], his wife, and his child concerning the oneness of God, the man would have an opinion, his wife would have a different one [than him], and his child would have a third opinion.”

Source: Al-Jawāb al-Saḥīḥ 2:299
Translated by: Riyāḍ al-Kanadī