With respect to the sinlessness of Jesus modern liberal historians find themselves in a quandary. To affirm that He was sinless means to relinquish much of that ease of defending liberal religion which the liberal historians are anxious to preserve, and involves hazardous assumptions with regard to the nature of sin. For if sin is merely imperfection, how can an absolute negation of it be ventured upon within a process of nature which is supposed to be ever changing and ever advancing? The very idea of “sinlessness,” much more the reality of it, requires us to conceive of sin as transgression of a fixed law or a fixed standard, and involves the conception of an absolute goodness. But to that conception of an absolute goodness the modern evolutionary view of the world properly speaking has no right.
At any rate, if such absolute goodness is to be allowed to intrude at a definite point in the present world-process, we are involved in that supernaturalism which, as will be observed later, is the very thing that the modern reconstruction of Christianity is most anxious to avoid. Once affirm that Jesus was sinless and all other men sinful, and you have entered into irreconcilable conflict with the whole modern point of view. On the other hand, if there are scientific objections, from the liberal point of view, against an affirmation of the sinlessness of Jesus, there are also very obvious religious objections against an opposite affirmation of His sinfulness–difficulties for modern liberalism as well as for the theology of the historic Church. If Jesus was sinful like other men, the last remnant of his uniqueness would seem to have disappeared, and all continuity with the previous development of Christianity would seem to be destroyed.
J. Gresham Machen, Christianity and Liberalism (Kindle Edition)