But, hey, Christians tell us Jesus had to die for our sins. A typical conversation might go something like this:
Monotheist: Oh. So you believe God died?
Trinitarian: No, no, perish the thought. Only the man died.
Monotheist: In that case, the sacrifice didn’t need to be divine, if only the man-part died.
Trinitarian: No, no, no. The man-part died, but Jesus/God had to suffer on the cross to atone for our sins.
Monotheist: What do you mean “had to”? God doesn’t “have to” anything.
Trinitarian: God needed a sacrifice and a human wouldn’t do. God needed a sacrifice big enough to atone for the sins of humankind, so He sent His only begotten son.
Monotheist: Then we have a different concept of God. The God I believe in doesn’t have needs. My God never wants to do something but can’t because He needs something to make it possible. My God never says, “Gee, I want to do this, but I can’t. First I need this certain something. Let’s see, where can I find it?” In that scenario God would be dependent upon whatever entity could satisfy His needs. In other words, God would have to have a higher god. For a strict monotheist that’s just not possible, for God is One, supreme, self-sufficient, the source of all creation. Humankind has needs, God doesn’t. We need His guidance, mercy and forgiveness, but He doesn’t need anything in exchange. He may desire servitude and worship, but he doesn’t need it.
Trinitarian: But that’s the point; God tells us to worship Him, and we do that through prayer. But God is pure and holy, and humankind are sinners. We can’t approach God directly because of the impurity of our sins. Hence, we need an intercessor to pray through.
Monotheist: Question—did Jesus sin?
Trinitarian: Nope, he was sinless.
Monotheist: How pure was he?
Trinitarian: Jesus? 100% pure. He was God/Son of God, so he was 100% holy.
Monotheist: But then we can’t approach Jesus any more than we can God, by your criterion. Your premise is that humankind can’t pray directly to God because of the incompatibility of sinful man and the purity of anything 100% holy. If Jesus was 100% holy, then he’s no more approachable than God. On the other hand, if Jesus wasn’t 100% holy, then he was himself tainted and couldn’t approach God directly, much less be God, the Son of God, or partner with God.