In contrast to modern education, Bob Heinlein's writings encouraged young men to be a polymath to meet the challenges of the future.

I believe a synonym for polymath would be the term 'a true Renaissance man or woman'.  Unfortunately, there are some today that would rather we slip back into the dark ages. 

Polymath comes from the Greek word polymathes, which means 'very learned' or 'having leaned much'.  It is deceptive, because at first glance you might assume it only means 'having leaned much mathematics' or poly 'meaning many - 'many mathematics', but that would be incorrect.  It has to do with much more than just one field of accomplishment.