In the chapter, “His Freedom From Sin,” Philip Schaff argued that Jesus could have sinned and did not have an “immaculate nature” (though, of course, he never did sin in his Christology!):
He was, indeed, tempted as we are; but he never yielded to temptation. [1] His sinlessness was at first only the relative sinlessness of Adam before the Fall; which implies the necessity of trial and temptation, and the peccability, or the possibility of sinning. Had he been endowed from the start with absolute impeccability, or with the impossibility of sinning, he could not be a true man, or our model for imitation: his holiness, instead of being his own self-acquired act and inherent merit, would be an accidental or outward gift, and his temptation an unreal show. As a true man, Christ must have been a free and responsible moral agent: freedom implies the power of choice between good and evil, and the power of disobedience as well as obedience to the law of God.
But here is the great fundamental difference between the first and second Adam: the first Adam lost his innocence by the abuse of his freedom, and fell, by his own act of disobedience, into the dire necessity of sin; while the second Adam was innocent in the midst of sinners, and maintained his innocence against all and every temptation. Christ’s relative sinlessness became absolute sinlessness by his own moral act, or the right use of his freedom in perfect active and passive obedience to God. In other words, Christ’s original possibility of not sinning, [2] which includes the possibility of sinning, but excludes the actuality of sin, was unfolded into the impossibility of sinning, [3] which can not sin because it will not. This is the highest stage of freedom where it becomes identical with moral necessity, or absolute and unchangeable self-determination for goodness and holiness. This is the freedom of God, and also of the saints in heaven with this difference, that the saints obtain that position by deliverance and salvation from sin and death, while Christ acquired it by his own merit. [4] (Philip Schaff, The Person of Christ: The Perfection of His Humanity Viewed as a Proof of His Divinity [London: James Nisbet and Co., 1888; repr., London: Windsor Avenue, 2015], 31-32, italics in original)
Notes for the Above
[1] Comp. with the history of the temptation in the wilderness, Matt. 4 and Luke 4, the significant passages in the Epistle to the Hebrews 4:15, “Tempted n all points as we are, yet without sin (πεπειρασμένον δὲ κατὰ πάντα καθ᾽ ὁμοιότητα χωρὶς ἁμαρτίας), and 5:8, "Though he was a son, yet learned he obedience by the things which he suffered” (καίπερ ὢν υἱός, ἔμαθεν ἀφ᾽ ὧν ἔπαθεν τὴν ὑπακοήν καὶ τελειωθεὶς ἐγένετο, κ.τ.λ.)
[2] In scholastic terminology, relative freedom from sin is called posse non peccare, or impeccabilitas minor. To this corresponds the posse non mori, or immortalitas minor, i.e., the relative or conditional immortality of Adam in Paradise, which depended on his probation and was lost by the Fall.
[3] The non posse peccare, or impeccabilitas major. With this is closely connected the non posse mori, or immortalitas major, the absolute immortality of the resurrection-state, which can never be lost.
[4] Dr. Jos. Berg (formerly professor in the Theological Seminary at New Brunswick), in a friendly notice of the first edition of this essay (in his “Evangelical Quarterly” for April 1861, p. 289), objects to this view of the peccability of the man Jesus, that it is inconsistent with his absolute holiness. But I cannot see the force of his objection. Peccability is merely the possibility of sin, such as attached also to Adam in the state of innocence; and it by no means involves Christ in the reality of sin, either original or actual. Against such an inference the language of the text is sufficiently guarded. It is true, the angel called Christ the Holy Thing from the moment of his conception, το γεννωμενον αγιον (Luke 1:35). But Adam was holy too, though “subject to fall” (as the Larger Westminster Confession expresses it, qu. 17). Moreover, this original holiness cannot exclude the idea of the physical and moral growth of the Christ-child; for this is distinctly asserted by the same Evangelist (Luke 2:40, 52; comp. Heb. 5:8). The denial of the possibility of sin overthrows the realness of Christ’s humanity, and turns the history of the Temptation into a Gnostic phantom and mere sham. It is just because Christ was really and actually tempted, and this not only by the Devil in the wilderness (Matt. 4), but throughout his whole life (Luke 22:28, Heb. 4:15), and because he successfully resisted the temptation under every form, that he became both our Saviour and our Example (comp. Heb. 5:7-9).
Commenting on incidents in the Gospels that some may appeal to in order to “prove” Jesus was guilty of personal sin (i.e., Jesus and the money chargers; Jesus cursing the fig-tree), Schaff wrote:
The apparent outbreak of passion in the expulsion of the profane traffickers from the Temple is the only instance on the record of his history which might be quoted against his freedom from the faults of humanity. But the very effect which it produced shows that, far from being the outburst of passion, the expulsion was a judicial act of a religious reformer, vindicating, in just and holy zeal, the honour of the Lord of the Temple. It was an exhibition, not of weakness, but of dignity and majesty, which at once silenced the offenders, though they were superior in physical strength, and made them submit to their well-deserved punishment without a murmur. They were over-awed by the presence of a superhuman power. The cursing of the unfruitful fig-tree can still less be urged; as it evidently was a significant symbolical act, foreshadowing the fearful doom of the impenitent Jews in the destruction of Jerusalem. On the contrary, these two facts become fully intelligible only by the assumption of the presence of the Divinity of Christ; for they represent him as the Lord of the Temple, and as the Lord of creation. (Ibid., 33-34)