What is the relation of God to the Pactum Salutis, the Covenant of Redemption? Some people might think of the Pactum Salutis as being predicated of God in Himself, God ad intra. But is that how we should be thinking of the Pactum?
When we think of God, we think of God in relation to Himself (ad intra) and God in relation to those external to Himself (ad extra). We think of God in His being, and God in His works. There is the immanent Trinity, and there is the economic Trinity. All of these distinctions are meant to clarify our thoughts concerning who God is, and distinguish the nature of God from what God actually does. God is immutable, and thus His nature is immutable. To predicate something about God ad intra, about His being, the immanent Trinity, is to predicate immutability to it. As God is simple, something predicated about God ad intra is predicated of His attributes, for God is His attributes.
Another philosophical/ theological category that is important for this discussion is that of necessity and contingency. Something is necessary because it cannot be otherwise. Something is contingent when it could be otherwise. Thus, for example, a man necessarily has a human genotype. But having the XY genotype is contingent for a man, because some men due to mutations do not have the XY genotype (e.g. XXY, XX with SRY gene attached).
In theology, God is a necessary being, because God cannot not exist. God's love to Man however is contingent, because God does not have to love anyone if He so chooses not to. God is necessarily omnipotent, because God cannot be not all powerful. Likewise, God is necessarily good, because God cannot ever be evil. Yet God's creating the world is contingent, because God does not have to create the world.
As we look at those examples, we can notice a pattern. Whatever that is of God's being, of God ad intra, of the immanent Trinity, is necessary. Whatever that is of God's works, of God ad extra, of the economic Trinity, is contingent. That must be the case, because God who is simple and necessarily existing implies that everything about the essence of the one God must be necessary. Conversely, the God who is sovereign does not need to do any of the works He has done (or could do), and therefore all of them are contingent.
So how should we think about the Pactum? Well, is the Pactum necessary or contingent? Does God have to make the Pactum, or does he not have to do so? Let's think for a moment. If the Pactum is predicated of God ad intra, then by virtue of simplicity, it must be necessary (and eternal, and immutable and so on). If it is necessary, then God cannot not make the Pactum. Since the Pactum speaks about created realities like "the cross," "death," and so on, even though they are spoken of in the abstract (since the world has not yet been created), then the concepts of these created realities must exist eternally. Moreover, the atonement of Christ then is necessary and eternal. Justification also becomes eternal, since justification is part of the Pactum (that the Son will save a people by justifying them). Also, since as we have mentioned many times, God is simple, that means God is to be identified with concepts of created realities, the atonement of Christ and justification!!?!
From this reductio ad absurdum, I hope it can be clearly seen that predicating the Pactum of God ad intra is untenable, at least not if one wants to be orthodox. The Pactum therefore must be predicated of God ad extra, and most definitely it must be contingent. If we maintain that God does not have to save anyone, then it must be the case that God does not have to provide salvation for sinners either, otherwise is God necessarily saving a set regardless of whether anyone is in it? But if salvation is a free (as opposed to necessitated) act of God, then by definition, it could be otherwise. And thus the Pactum as a free act of God must be able to be otherwise also.
The Pactum therefore must be and only can be predicated of God's works, of God ad extra, of the economic Trinity. But... the Pactum is made in eternity? And that is why we speaks of submission of the Son from eternity, understanding this "eternity" not as the timeless eternity predicated of God's being, but of the everlasting eternity predicated of God's works. From my perspective, if one is to be consistent with the various facets of one's theology, there is simply no way to make the Pactum as speaking of the immanent Trinity without creating the type of absurdities (eternal forms of created realities, eternal atonement, eternal justification; in the being of God) that we have shown.