Neither Christians nor Jews (in general) believe in the Oneness of Allah.
They (Jews and Christians) took their rabbis and their priests to be their lords besides Allâh … (Qur’an 9:31)
Elaborating on this verse, the Prophet (pbuh) remarked:
“They did not worship their rabbis and their monks. However, they believed in what was declared lawful or unlawful by them, though without any sanction.” (Tirmidhi)
Allah charges them in the above verse, with taking their rabbis and their monks as their lords besides Allah. The Prophet (pbuh) explained this point with reference to their blind acceptance of their pronouncements about lawful and unlawful things. They never examined their ruling in the light of the Torah or the Gospel. In other words, they blindly followed them.
By obeying their priests in things which they made lawful or unlawful according to their own desires without being ordered by Allah, they made them partners of Allah.
Also, the Qur’an says:
{And thou wilt surely find that, of all people, those who say, “Behold, we are Christians”, come closest to feeling affection for those who believe in this divine writ. This is so because there are priests and monks among them, and because they are not given to arrogance.
For, when they come to understand what has been bestowed from on high upon this Apostle (Muhammad), thou canst see their eyes overflow with tears, because they recognize something of its truth; and they say: “O our Sustainer! We do believe; make us one, then, with all who bear witness to the truth.”} … (Al-Ma’idah 5:82-83)
And so, if Jews and Christians refuse to accept Muhammad (pbuh) as the Messenger of Allah, then they need to ask themselves who are they following i.e. their scriptures OR their church leaders and rabbis.
Again, we need to remember this verse of the Qur’an:
They (Jews and Christians) took their rabbis and their priests to be their lords besides Allâh … (Qur’an 9:31)
The Messenger of Allah (pbuh) said:
“By the One in Whose hand is the soul of Muhammad, there is no-one of this Ummah, Jew or Christian, who hears of me then dies without believing in that with which I have been sent, but he will be one of the people of Hell.” (Sahih Muslim, 153).