Classical art and discourse takes place in a network of symbols & ideas nearly completely lost to us. In art created less than 400 years ago, I'd say less than 5% of the content is accessible to your average university-educated person. Imagine in the case of archaic Greece.
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This means there are serious shortcomings to just picking up Plato or Aquinas and having a go at it?
Such material is simply not accessible for an author like Plato; you can read and study all that was written before him in philosophy in less than a year. I'm not too familiar with Aquinas' time, so I can't really say.