The lapsarian debate has produced what Kant calls “an antimony:” two seemingly true positions which cancel out each other. The supralapsarian is correct in that what is first in intention is last in execution. I don’t know if this is a universal maxim, but I suspect it is. There is no getting around that. The infralapsarian is correct that when God’s decree seems to follow his creating the “one for dishonor, the other for honor, from the same lump” (Romans 9, so Hodge and Turretin).
I think supralapsarianism has the edge, but not in the way the discussion usually goes. On anyone’s doctrine of God, God is simple and his eternal knowledge is immediate and non-discursive. God doesn’t decide to do this and then do that. While the infra is correct that Paul has God using a lump of clay prior to the decree to save/damn, I wonder if Paul is merely using that as an illustration and nothing more.
I have not seen most Reformed people synthesize their correct understanding of God’s knowledge with election and incarnation. The result, when done, is something like this: If God’s knowledge is immediate and non-discursive, which all but Eastern Orthodox and Jesuits will acknowledge, then we may not say that God first decides to create and then decides to elect, or vice-versa. Reformed people know this, but they are not as aware that this failure creates a metaphysical “gap” in the being of God. As McCormack notes,
So the event in which God constitutes himself as triune is identical with the event in which he chooses to be God for the human race. Thus the ‘gap’ between ‘the eternal Son’ and ‘Jesus Christ’ is overcome, the distinction between them eliminated…. There is no ‘eternal Son’ if by that is meant a mode of being in God which is not identical with Jesus Christ” (pp. 218-19)
If God has always decided to be God-in-Christ, then he must have always been God-in-Christ-for-his-people. This is the heart of supralapsarianism.
Of course, there are some problems due to the anthropomorphizing in any language, but I think it holds up. However, I am not saying that the incarnation is eternal nor am I saying that Christ would have come regardless of Adam’s fall (I believe the opposite, actually). Theologians make a distinction between God’s decree and the historical outworking of God’s decree, without an imputing temporality into the eternal Godhead, and so that is how I would say that I don’t believe in an eternal incarnation.