I believe with Justin Martyr, that all good men are Christians, and I believe there have been, and are, good men in all nations, sincere and conscientious.

John Adams (1735–1826) American lawyer, Founding Father, statesman, US President (1797–1801)
Letter (1820-07-08) to Simon Miller
    (Source)

Justin Martyr (c. AD 95–165) was an early Christian apologist and philosopher. He argued that many historical Greek philosophers, including Socrates and Plato, taught Christian virtues and were thus unknowingly Christian.