Because I knew: therefore I gave thee the more and clearer demonstrations of my Divine nature and providence, because I knew thou wast an unbelieving and perverse nation, that would not easily nor willingly be convinced. Thy neck is an iron sinew, which will not bow down to receive my yoke, nor to obey my commands. It is a metaphor taken from untamed and stubborn cattle; of which see also Nehemiah 9:29 Malachi 7:11 Acts 7:51. The sense is, I considered that thou wast unteachable and incorrigible. Thy brow brass; thou wast impudent, and therefore wouldst boldly pretend that thou didst forsake me, for want of full conviction of my Divine authority, and of thy duty; therefore I determined that I would leave thee without excuse.
Related Commentaries of Isaiah 48:4
Isaiah 48:4
4 Because I knew that thou art obstinate,a and thy neck is an iron sinew, and thy brow brass;