The point is that a hidden sign is proved by a manifest one. Commentary:
Psuedo-Chrys., Vict. Ant. e Cat. in Marc.: And because it is easier to say than to do, there was still manifestly something to say in opposition, for the work was not yet manifested.
Wherefore He subjoins, “But that ye may know, &c.” as if He said, Since ye doubt My word, I will bring on a work which will confirm what was unseen.
But He says in a marked manner, “On earth to forgive sins,” that He might shew that He has joined the power of the divinity to the human nature by an inseparable union, because although He was made man, yet He remained the Word of God; and although by an economy He conversed on the earth with men, nevertheless He was not prevented from working [p. 41] miracles and from giving remission of sins.
For His human nature did not in any thing take away from these things which essentially belonged to His Divinity, nor the Divinity hinder the Word of God from becoming on earth, according to the flesh, the Son of Man without change and in truth.
Theophylact: Again, He says, “Take up thy bed,” to prove the greater certainty of the miracle, shewing that it is not a mere illusion; and at the same time to shew that He not only healed, but gave strength; thus He not only turns away souls from sin, but gives them the power of working out the commandments.
Bede: A carnal sign therefore is given, that the spiritual sign may be proved, although it belongs to the same power to do away with the distempers of both soul and body.
Whence it follows: “And immediately he arose, took up the bed, and went forth before them all.”
Chrys.: Further, He first healed by the remission of sins that which He had come to seek, that is, a soul, so that when they faithlessly doubted, then He might bring forward a work before them, and in this way His word might be confirmed by the work, and a hidden sign be proved by an open one, that is, the health of the soul by the healing of the body.