I used to use the argument "Israel as an ethnic-based exclusively Jewish state is a throwback to earlier centuries, and modern states are secular and non-exclusive." I used it until someone completely tore it to pieces. He was right.
The issue isn't that the state of Israel is a good thing or not. It definitely was illegitimately created by expelling the majority Palestinian population and creating a Jewish ethnocracy. It's also an expansionist Western colonial settler-state based on theft and oppression (not just in the West Bank but from 1948 on). But don't use the “not a modern state” argument.
Unfortunately, ethnic states are common these days. Look at Serbia. How about Belgium which is on the bring of splitting apart into Francophone and Dutch sections. Remember Czechoslovakia? Then there's the Hindi Tamals who fought for independence from the Buddhist Sinhalese majority in Sri Lanka.
Going back to the 18th and 19th centuries you can find multi-ethnic states, most notably the double kingdom of Austria-Hungary. Switzerland has been an official tri-ethnic state for centuries. Going back to antiquity you can find a lot of multi-culti states especially the Roman Empire, or Persian.
So Israel is not a throwback in that sense. It is a hanger-on from the past in terms of being the last significant European (including American and Canadian Jews whose ethnicity includes Central and Eastern European Jewish ancestry) colonial settler-state.