The Israelites had concluded that Yahweh was blind and deaf to their situation, namely, impending destruction. Now He revealed that it was they who were blind and deaf to what He would do for them. He challenged them to comprehend what they had missed. [source][source][source]
Context Summary
Isaiah 42:14-25 - A Deaf And Blind Messenger
There are times in our lives when God seems to hold His peace. Evil is rife, bad men prosper, society lies under the spell of vice. It is only temporary, however. Then God comes forth out of the silence, and shows Himself "strong in the behalf of them whose heart is perfect toward Him." He brings the blind "by a way that they knew not," and makes "the crooked places straight."
These wonderful things are wrought not for the wise and holy alone, but for the blind and the deaf, who nevertheless desire to serve Him. See Isaiah 42:19. God's help is not conditioned by our merit, but by our faith. In the eyes of men we may be the least fit to claim divine succor. But our deficiencies and failures constitute our most eloquent claim-God knew what we were, before He ever stooped to identify Himself with us. He is pleased to help us "for His righteousness' sake." His name and character must be maintained. Therefore He has magnified the law and made it honorable by the matchless obedience and death of His only begotten Son. See Galatians 4:4-5. [source]
Chapter Summary: Isaiah 42
1The office of Christ, graced with meekness and constancy 5God's promise unto him 10An exhortation to praise God for his Gospel 13God will manifest himself, and check idolatry 18He reproves the people of incredulity
What do the individual words in Isaiah 42:18 mean?