“This sinlessness of our Lord, however, does not amount to absolute impeccability. It was not a non potest peccare. If He was a true man He must have been capable of sinning. That He did not sin under the greatest provocation; that when He was reviled He blessed; when He suffered He threatened not; that He was dumb, as a sheep before its shearers, is held up to us as an example. Temptation implies the possibility of sin. If from the constitution of his person it was impossible for Christ to sin, then his temptation was unreal and without effect, and He cannot sympathize with his people.” Systematic Theology, III.iv.2.2
The Sovereign Logos 