The Westminster Standards (Confession of Faith, Shorter Catechism and Larger Catechism) are the home of a theology as profound as it is clear and precise. This may seem confusing to some, who think that if you can speak with plain forthrightness the matter must be simple, that if something is expressible it is consequently jejune. The ineffable does not lose its quality of mystery when we state with comprehensible precision everything that can be said about it. For though vagueness and mist are sometimes confused for depth, the real marvels can be set out plainly in the full light of day and still boggle the mind.
One area where this profundity is evident is in the doctrine of the covenants. WCF VII,1 says:
The distance between God and the creature is so great, that although reasonable creatures do owe obedience unto him as their Creator, yet they could never have any fruition of him as their blessedness and reward, but by some voluntary condescension on God’s part, which he hath been pleased to express by way of covenant.
The ontological gulf, the distance between God and man with regard to being, was bridged not by man ascending a ladder to God, not by a series of emanations, and not by the Incarnation of Christ. The ontological gulf between God and man was bridged by God’s voluntary condescension – by the covenant of works. When man fell into sin, and rendered himself uncapable of life by that covenant (VII,3), there was a new gulf. To the distance of being between God and man, there was added a moral distance. The covenant of grace, the appointment of Christ as Mediator with all that entailed, including the Incarnation, was the bridge of that moral gulf, so that God could again dwell in and among His people. The Incarnation was never meant to bridge the ontological gulf between God and man. Thus the Confession goes on to assert (VIII,2):
“…two whole, perfect and distinct natures, the Godhead and the manhood, were inseparably joined in one person, without conversion, composition or confusion.”
The Incarnation does not render us ontologically capable of union with God, or of the knowledge of God. There is never any possibility of some hybrid of deity and humanity: the natures cannot be blended: the ontological distance never disappears, though God accommodate Himself to us. The One who dwells in inaccessible light must still approach us: if we are to know Him at all He must condescend to our low estate, to our near approach to non-being, to our paltry ontological status. But the Incarnation, as part of what was required for the Son to be Mediator between God and man, bridges the moral gulf – the distance between our wickedness and God’s holiness, so that our vileness is no longer an obstacle to God’s purpose of condescension.
That means, of course, that the covenant of grace is superior to the covenant of works: in the latter, God kindly overcame a natural distance to make Himself known to us; but in the former, God graciously overcomes not only a natural but a moral distance to make Himself known to us as not only our Sovereign Lord but as also our Loving Redeemer.
That means also that salvation should not be conceived of in ontological categories. This would be sufficiently evidenced by God’s pronouncement of His whole creation, including man, to be very good. We don’t need deliverance from a condition of exceeding goodness. It is also manifest in the Incarnation itself. If being human were intrinsically wrong or evil, the Son of God could never have taken into personal union with Himself a complete human nature. Christ came, not to deliver us from the condition of humanity, but from humanity’s accidental (in the sense of not proper to its essence) bondage to sin and consequently the devil and death.
And together all of that means that the attempt to find some natural ground in God that required the Incarnation, to insist with some absurd speculators that Christ would have become incarnate even had the Fall never happened, is ultimately profane.
To begin with, it means prying into what has not been revealed, and in so doing that speculation reveals a lack of proper humility before God (Psalm 131).
This notion fails, secondly, in that it does not recognise God’s providence over all, which requires us to believe that the Fall was included in God’s decree, and though contingent as to Adam, infallibly certain as to God.
It fails too because it seeks for something higher than the will of God, for a cause of God’s will. It looks for a necessity planted in the divine nature, so that the Incarnation is not a free determination of God’s will to deal with the problem of sin, but an inevitable expression of some thing essential in God. This is a fundamental mistake, since there is nothing higher than the will of God, and for His will no cause may be assigned.
In addition to the preceding points, which demonstrate that such speculations minimise God’s sovereignty and the perfection of His decree, it should be said, as sufficient for simple souls, that believing Christ would have become incarnate without sin also falsifies the Scriptural representations that Christ came into the world to save sinners (e.g., 1 Timothy 1:15, 1 John 4:9,10). This wasn’t a super-added purpose, an additional benefit to proceeding with something that was already going to take place. This was the reason that the Son was united to a human nature, and took upon Himself our curse and our subjection to the law.
And the ultimate effect is to minimise and dishonour the grace of God. It diminishes God’s grace to argue that the Incarnation was not done graciously, freely, but because of an intrinsic compulsion: obscuring that this act of condescension is voluntary robs God of the praise due to Him for it. It dishonours the grace of God by failing to recognise that it was on account of sin that this staggering act of condescension was undertaken; it was not man as man, in his condition of humanity, but man as sinful -vile, rebellious, hateful- upon whom God had mercy in sending His Son to be born of a woman.
 For more on this point, B.B. Warfield’s article “The Principle of the Incarnation” in Selected Shorter Writings, v.1 is a good beginning point.
 For more on this point consult Thomas Aquinas’ Summa Theologica Part 1, Question 19, Articles 4 & 5 and Francis Turretin’s Institutes of Elenctic Theology, Topic 3, Question 17.