Could Israel have known who Jesus was?
Yes, Israel could have and should have known who Jesus was when He came to the nation, according to Scripture. However, despite that opportunity and responsibility, Israel did not know Him. The context states they could have known, should have known, but did not.
Les explains that Paul returns to Israel's situation in verse 18, asking whether they had not heard and whether Israel did not know. Les teaches that when Jesus came to the nation of Israel, they could have known who He was and should have known who He was, yet Israel did not know. The passage notes their sound went into all the earth and their words to the ends of the world, but Israel still did not know.
But I say, Did not Israel know? First Moses saith, I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you.