St. Anatoly of Optina 1824-1894
Do not fear sorrows, but fear the stubbornness of heretics who try to separate a man from Christ, which is why Christ commanded us to consider them as pagans and pharisees. (Living Without Hypocrisy, p. 48)
St. Nektary of Optina 1853-1928
One may have a good worldly relationship with nonbelievers, but one cannot have a relationship in prayer and one must not carry on arguments about religion so that the name of God not be offended during an argument. (ibid., p. 98)