[OrthodoxPresbyterian]

"What God commands man is able to do". ~~ Why wouldn't you think it sound?

God commands all men to Obey the Law of God perfectly.

Is the natural, unregenerate man able to do this?

[jboot]

"What God commands man is able to do"

VS

"God commands all men to Obey the Law of God perfectly."

OP is on to something. The former makes the Cross of Christ of no account.

Or is the sacrifice of Jesus "Plan B", initiated by God after no men chose to obey His first plan?

[jude24]

Non posse non peccare has been the orthodox Christian viewpoint for almost 2 millennia.

Heck -- even my Arminian professor will affirm non posse non peccare.