Unitarians famously quote the passages in Isaiah 44-66, commonly referred to by scholars as a polemic against the pagan nations, to stress God’s singularity, but they miss the point of the polemic: The prophet is not so much concerned about ontology as He is about exclusive worship to Yahweh. He is emphasizing that only Yahweh is worthy of worship, and is engaging in a polemic against syncretism.
Read Isaiah 44 and 45, and not particularly 43:12, where God says, “I have declared and saved, I have proclaimed and there was no foreign gods among you.” God, through the prophet Isaiah, is attacking idolatry. The true God, Yahweh, is being contrasted with the false gods of the surrounding nations. To use this passage as through Isaiah was dealing with the nature of God is absolutely absurd. (From pp. 14-15 of the booklet, Is Jesus Really God? Published by The Church of God International)