Fred has posted a great argument, which he is not yet willing to embrace, about the Trinity and answers to prayer. Now I am a Trinitarian (said as I piously cross myself) and I greatly admire Andrew Murray, but I think the argument against radical monotheism (such as Islam) might fail on the following grounds:
Suppose God is outside of time (experiencing everything in human time as a “now”) and has foreknowledge. Such a God could be said to “answer in prayer” in the single creative act that produced the world. Knowing what He knew about the future as He created the world He also met those needs in it that fulfilled His divine desires. If God could create as a radically monotheistic entity, then He certainly could make that single creative act answer all His desires (which would include making those states of affairs exist that He willed to exist.) Such a God heard the prayers we will make from the foundation of the world and so does not change His mind (which seems to be Murray’s fear), but made up His mind from the start. From that perspective God does not answer our prayers, He has already answered our prayers. The relationship with us formed in the act of creating us seems sufficient to me to spur the Divine will in this timeless creation. This also seems to mean prayer matters, our freely praying (assuming free will is compatible with foreknowledge) is built into the entire plan for the cosmos.
The Trinity, it seems, adds a poetic beauty to answered prayer as it has God responding to us the way He responds within Himself, but I don’t think it improves on radical monotheism in the way Murray hopes.
But perhaps I am missing something in Murray’s argument. . .