Speak with conscience. The next sentence is

Speaking with conscience is the truest expression; Acting according to conscience is the best way.

Speaking with your conscience means that what you say should be worthy of your conscience.

Conscience, Chinese vocabulary, Confucian nouns. Interpretation: it is a code of conduct and value standard generally recognized by the real society and recognized by itself. Conscience is the basic form of moral emotion and the outstanding embodiment of personal self-discipline.