The context is Enlightenment thinkers wishing to embrace the pagan writer Celsus. Celsus's Platonism was not popular in the Enlightenment, since the Platonists were considered too mystical and irrational. So labelling Celsus a covert Epicurean meant the Enlightenment thinkers could read his works with a more open mind.
The sentence I have so far is
Celsus was held to be an Epicurean who did not lay his cards on the table.
I'm looking for something in English -- if it exists -- closer to the German idiom 'to fight with an open visor' (mit offenen Visier kämpfen). Failing that I would also be interested in other similar idioms.