I’ve been harping on modalism about the Son for a while now, and I’ve even given what I think is a convincing argument against it. Still, what if it is false? Why do I care? Why do I think it matters?
Remember, the claim is that the Son just is God, or else is a mode of God, a way God is. I think that S modalism has a couple of very unacceptable consequences. First, the theoretical ones. If S modalism is true, then:
1. The New Testament writers are either incompetent metaphor users, or they’re just mistaken about Jesus and God. Jesus is described in the NT as a “priest” and as the “Son of God”. Those metaphors would simply be inappropriate if Jesus is numerically identical to God, or is a mode of God. Again, the Son is supposed to me a “mediator” between God and the rest us. But if he just is God, that’s to say that there doesn’t need to be a mediator. Or if a mode of God is the “mediator”, again, there’s no need for one. The idea there would just be that God has a friendly, approachable side. So if S modalism is true, the NT writers who use that metaphor as either ill-informed, or are incompetent metaphor users. I resist both conclusions. Why? Because the NT writers were all, directly or indirectly, taught by Jesus, and I hold that Jesus was a competent teacher.
2. If Jesus is a mode of God, then he is not a human. Every human is a substance/entity, and no substance is a mode of any other substance, so no mode of anything is a human. And if Jesus is numerically identical to God, we combine that with the truth that God isn’t (identical to any) human being. Ergo, if S modalism is true, Jesus isn’t a human being. The three-dollar name for this is docetism, the idea that Jesus only seemed human. I take it that this is inconsistent with the New Testament, as well as with other things that most Christians want to say about Jesus. Yes, I know – “two natures” and so on. I going to pass that by for the moment; in the future we’ll chew on that at some length.
For now let me pass on the practical consequences of S modalism:
3. Jesus can’t be a mediator between God and us, but we do need such a mediator – the holiness and glory of God demand it. How about Mary? Or maybe, a priest? Maybe a patron saint? So much for the Pauline vision of Christians accessing God through Christ, a kingdom filled with “priests” with no need of other, merely human intermediaries. I suggest that S modalism was a big factor in the development of the cult of Mary, the manifold forms of saint-worship (honoring – whatever), and the strongly bishop- and later priest- oriented direction that proto-Catholocism took in the early middle ages. (Conversely, if you believe that Jesus is your “advocate” with the Father, then all these other suggested intermediaries are superfluous. But this requires S modalism to be false.)
4. Again, if Jesus just is God, or is a mode of God, hence, Jesus can’t realistically be a model for us to imitate. We’re humans, and we need a genuinely human example to imitate. We’d better find someone else… How about, I don’t know, this guy?
Well, I’m off to preach to the birds now… 
But first, how do you know if someone is a S modalist? In a church context, the main way is this – they throw around the terms “Father”, “God”, “Jesus” with no discernible rhyme or reason. They may start a prayer with “Father”, address the object of prayer two sentences later as “Jesus”, then end with “in your [Jesus’? The Father’s?] name…” If those three terms refer to one and the same thing, then there’s no point in being picky about how they’re used.