Another objection is that the Trinity does not appear in the earliest revealed texts, those of early Jewish tradition (traditionally held to be the Pentateuch, but the objection could work in a different arrangement of dating). And the exegetical principle to which Ibn Taymiyyah appeals is that newer texts must be interpreted on the basis of older texts.
This objection raises an important criterion for authenticity of later revelations. However, it does not avail to debunk the doctrine of the Trinity.
If God knows all things, he cannot be deceived. And if he is all good, he cannot deceive. He is both omniscient and omnibenevolent; therefore, he cannot be deceived nor deceive. Therefore, he would not reveal something false to begin with and then later correct an error. Therefore, all revelation is true. Therefore, no later revelation can contradict an earlier revelation. Therefore, that which does contradict an earlier revelation is false. This much is true.
However, qualifications need to be made. Muslims recognize that some texts and revelations were originally meant only for a period of time. These can be “abrogated”. Christians believe the same. For instance, the “Old Law” was abrogated by the “New Law” of Jesus Christ. That means the ceremonies and political precepts of the Old Law had their use only for a time; Christ abolished them all by his sovereign authority. He laid down one New Law, which incorporated the moral precepts of the Old Law (those which correspond to natural law and some which correspond to divine law). Thus, on this score Christians and Muslims are on similar pages. In addition, Christians agree that no “allegedly revealed truth claim” that comes later can be true if it contradicts an earlier revealed truth claim. This is because truth is eternal.
However, as Christians will argue, the Old Testament revelation that God is one does not contradict the New Testament revelation of distinct divine persons. We will need to argue this out. And I will. However, note how the Islamic objection on this principle fails. Taymiyyah insisted that unless the proposition (Q) is revealed in the earliest books, it cannot be revealed in the later. This is manifestly absurd. It implies that God must say all things in his first statement. But, as Jews believe, so do Christians believe: God speaks in the language of men! And our human language cannot bear all truth in one statement. Nor is truth limited to one propositional statement. Truth is rich, because our minds are small. And therefore, we need to state many things about Truth in order to suggest this richness to our poverty. Therefore, if God speaks in the language of men – and if not, who is listening??? – then he speaks gradually, over time, revealing more and more of his High Truth to our lowliness. Thus, the principle Taymiyyah lays out and by which he purports to devastate Trinitarian doctrine, is utter nonsense.
Finally, I would note that if his principle were correct, then the Qur’an could add nothing to the first sentence of the Bible. But presumably Taymiyyah thinks the Qur’an does add something. Ergo….