ALL Scripture is inspired not just the New. There is no other name than Jesus by which people are saved. Those in the Old were saved by the grace Jesus merited for us by his sacrifice just as we are.I hate to be the one to have to point out the obvious, but Isaiah was written to the Jews who were under the Old Covenant. Under the Old Covenant, people were not saved as they are under the New Covenant.
Just as they could choose God, we can choose God.
Every man can seek God and find him.
"And as he made from one man every nation of mankind to live on all the face of the earth, having determined allotted periods and the boundaries of their dwelling place,that they should seek God, and perhaps feel their way toward him and find him" Acts 17:26-27