Isaiah 8:16

Bind up the testimony— Engrave the testimony, seal up the law in my disciples. There seems no doubt that the person here introduced speaking is God the Father, in whose will the Messiah immediately after professeth his acquiescence. The testimony and law, mean the doctrine and the law of the Messiah, at which many should be offended; and the words are frequently used in the prophets and the sacred writers for the doctrine of Christ. The engraving or sealing of this in men, is that efficacious operation of Christ and his Spirit, whereby men are brought to a belief of, and an obedience to the word of truth written in their hearts. See Jeremiah 33:2

Cor. Isaiah 3:3 and 1 Corinthians 1:6 in which last passage the apostle seems to have had his eye on these words of Isaiah, as also in 2 Corinthians 1:22 and Ephesians 4:30.