To give an example of what I mean, I've finally gotten around to reading John Owen at length, which I had been meaning to do for some time. You can read his work on the Priesthood of Christ here, though I bought the paperback which smooths out some of the eyesores. While I plan to write on what he has to say about the priesthood of Christ, I came across an excellent section in which Owen takes Socinians to task for suggesting that the incarnation of the Son would have occurred even if man had never sinned. I think it's a good illustration of why I think that counter-factual reasoning in particular is dubious:
10. As to the first opinion, concerning the incarnation of the Son of God without respect unto sin and redemption, there are many pretences given unto it, which shall be afterwards particularly considered. They say that “the manifestation of the glory of God required that he should effect this most perfect way of it, that so he might give a complete expression of his image and likeness. His love and goodness also were so perfectly to be represented, in the union of a created nature with his own. And herein, also, God would satisfy himself in the contemplation of this full communication of himself unto our nature. Besides, it was necessary that there should be a head appointed unto the whole creation, to conduct and guide it, man especially, unto its utmost end.” And sundry other things they allege out of the Bible of their own imaginations. It is granted that even in that state all immediate transactions with the creatures should have been by the Son; for by him, as the power and wisdom of God, were they made, John 1:3; Hebrews 1:2; Colossians 1:16,17. He, therefore, should have immediately guided and conducted man unto his happiness, and that both by confirming him in his obedience and by giving him his reward; an express document whereof we have in the angels that sinned not. But for the opinion of his being incarnate without respect unto redemption and a recovery from sin and misery, the whole of it is ἄγραψον, or unwritten, and therefore uncertain and curious; yea, ἀντἰγραψον, or contrary to what is written, and therefore false; and ἂλογον, or destitute of any solid spiritual reason for the confirmation of it.
11. First, It is unwritten, — nowhere revealed, nowhere mentioned in the Scripture; nor can an instance be given of the faith of any one of the saints of God, either under the old testament or the new, in this matter. The first promise, and consequently first revelation, of the incarnation of the Son of God, was after the entrance of sin, and with respect unto the recovery of the sinner, unto the glory of God. Hereby are all other promises, declarations, and revelations concerning it, as to their end, to be regulated; for that which is the first in any kind, as to an end aimed at, is the rule of all that follows in the same kind. And therefore that which men ground themselves upon in this opinion is indeed neither argument nor testimony, but conjecture and curiosity. They frame to themselves a notional state of things, which they suppose beautiful and comely, (as who are not enamored of the fruits of their own imaginations?) and then assert that it was meet and according unto divine wisdom that God should so order things unto his own glory as they have fancied! Thus they suppose, that, without respect unto sin or grace, God would take unto himself the glory of uniting our nature unto him. Why so? Because they find how greatly and gloriously he is exalted in his so doing. But is this so absolutely from the thing itself, or is it with respect unto the causes, ends, effects, and circumstances of it, as they are stated since the entrance of sin, and revealed in the Scripture? Setting aside the consideration of sin, grace, and redemption, with what attends them, a man may say, in a better compliance with the harmony and testimony of Scripture, that the assumption of human nature into union with the divine, in the person of the Son of God, is no way suited unto the exaltation of divine glory, but rather to beget false notions and apprehensions in men of the nature of the Godhead, and to disturb them in their worship thereof; for the assumption of human nature absolutely is expressed as a great condescension, as it was indeed, Philippians 2:5-8, and that which served for a season to obscure the glory of the Deity in him that assumed it, John 17:5. But the glory of it lies in that which caused it, and that which ensued thereon; for in them lay the highest effects and manifestations of divine love, goodness, wisdom, power, and holiness, Romans 3:24-26. And this is plainly revealed in the gospel, if any thing be so. I fear, therefore, that this curious speculation, that is thus destitute of any scriptural testimony, is but a pretense of being wise above what is written, and a prying into things which men have not seen, nor are they revealed unto them.
12. Secondly, This opinion is contradictory to the Scripture, and that in places innumerable. Nothing is more fully and perspicuously revealed in the Scripture than are the causes and ends of the incarnation of Christ; for whereas it is the great theater of the glory of God, the foundation of all that obedience which we yield unto him, and of all our expectation of blessedness with him, and being a thing in itself deep and mysterious, it was necessary that it should be so revealed and declared. It were endless to call over all the testimonies which might be produced to this purpose; some few only shall be instanced in. First, therefore, On the part of the Father, the sending of the Son to be incarnate is constantly ascribed unto his love to mankind, that they might be saved from sin and misery, with a supposition of the ultimate end, or his own glory thereby: John 3:16, “God so loved the world, that he gave his only-begotten Son, that whosoever believeth in him should not perish, but have everlasting life.” Romans 3:25, “Whom God hath set forth to be a propitiation.” Chap. 5:8, “God commendeth his love toward us, in that, while we were yet sinners, Christ, died for us.” Chap. 8:3, “For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh.” 1 John 4:9; Galatians 4:4, 5. Secondly, On the part of the Son himself, the same causes, the same ends of his taking flesh, are constantly assigned: Luke 19:10, “The Son of man is come to seek and to save that which was lost.” 1 Tim. 1:15, “This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners.” Hebrews 2:14, “Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil.” Galatians 2:20; John 18:37, “To this end was I born, and for this cause came I into the world, that I should bear witness unto the truth,” — namely, of the promises of God made unto the fathers concerning his coming; Romans 15:8. See Philippians 2:6-11. And all this is said in pursuit and explication of the first promise concerning him, the sum whereof was, that he should be manifested in the flesh to “destroy the works of the devil,” as it is expounded 1 John 3:8. This the whole Scripture constantly and uniformly giveth testimony unto, this is the design and scope of it, the main of what it intends to instruct us in; the contrary whereunto, like the fancying of other worlds, or living wights in the moon or stars, dissolves the whole harmony of it, and frustrates its principal design, and therefore is more carefully to be avoided than what riseth up in contradiction unto some few testimonies of it. I say, that to ascribe unto God a will or purpose of sending his Son to be incarnate, without respect unto the redemption and salvation of sinners, is to contradict and enervate the whole design of the revelation of God in the Scripture; as also, it riseth up in direct opposition unto particular testimonies without number. Origen observed this, Hom. xxiv. in Numer.:
“Si non fuisset peccatum, non necesse fuerat Filium Dei agnum fieri; sod mansisset hoc quod in principio erat, Deus Verbum. Verum quoniam introiit peccatum in hunc mundum, peccati autem necessitas propitiationem requirit, propitiatio vero non sit nisi per hostiam, necessarium fuit provideri hostiam pro peccato;”
— “ If sin had not been, there would have been no necessity that the Son of God should be made a lamb; but he had remained what he was in the beginning, God the Word. But seeing that sin entered into the world, and stood in need of a propitiation, which could not be but by a sacrifice, it was necessary that a sacrifice for sin should be provided.” So Austin,Serm. 8 de Verbis Apostoli, tom. x., “Quare venit in mundum peccatores salvos facere. Alia causa non fuit quare veniret in mundum.”
13. Thirdly, This opinion is destitute of spiritual reason, yea, is contrary unto it. The design of God to glorify himself in the creation and the law or covenant of it, and his design of the same end in a way of grace, are distinct; yea, they are so distinct as, with reference unto the same persons and times, to be inconsistent. This our apostle manifests in the instance of justification and salvation by works and grace:
“If it be by grace, then it is no more of works: otherwise grace is no more grace. But if it be of works, then it is no more grace: otherwise work is no more work,” Romans 11:6.
It is impossible that the same man should be justified by works and grace too. Wherefore God, in infinite wisdom, brought the first design, and all the effects of it, into a subordination unto the later; and so he decreed to do from eternity. There being, by the entrance of sin, an aberration in the whole creation from that proper end whereunto it was suited at first, it pleased God to reduce the whole into a subserviency unto the design of his wisdom and holiness in a way of grace; for his purpose was to reconcile and gather all things into a new head in his Son, Jesus Christ, Ephesians 1:10; Hebrews 1:3, 2:7, 8. Now, according to this opinion, the incarnation of the Son of God belonged originally unto the law of creation, and the design of the glory of God therein. And if this were so, it must, with the whole old creation and all that belonged thereunto, be brought into a subordination and subserviency unto the succedaneous design of the wisdom of God to glorify himself in a way of grace. But this is not so, seeing itself is the fundamental and principal part of that design. “Known,” indeed, “unto God are all his works from the beginning.” Therefore, this great projection of the incarnation of his Son lying in the counsel of his will from eternity, he did, in wisdom infinite and holy, order all the concernments of the creation so as they might be disposed into an orderly subjection unto his Son incarnate. So that although I deny that any thing was then instituted as a type to represent him, — because his coming into the world in our flesh belonged not unto that estate, — yet I grant things to have been so ordered as that, in the retrieval of all into a new frame by Jesus Christ, there were many things in the works of God in the old creation that were natural types, or things meet to represent much of this unto us. So Christ himself is called the “second Adam,” and compared to the “tree of life,” whereof we have discoursed in our exposition on the first chapter.