Matthew Poole, A Seasonable Apology for Religion, pp. 27-28:
Thirdly, learn hence to maintain the honour of Religion. If wicked men's mouths be open against it, let not your mouths be shut, but open, for the defense of it. Take heed that no less dangerous, than common, sin of being ashamed of Religion; remembering that fever, but righteous, sentence of our Saviors, Whososever shall be ashamed of me, and my words in this adulterous and sinful generation, of him also shall the Son of man be ashamed, when he cometh in the glory of his Father, with the holy Angels. Mark 8.38. and Mat. 12.30. He that is not with me, is against me: and he that gathereth not with me, scattereth abroad.
It is a part of the Prophets charge against the Jews, that they are not valiant for the truth upon the earth: Jer. 9.3. Wicked men are not ashamed to wear the Devil's Livery, and why should you be ashamed to wear God's? I beseech you content not your selves with owning Religion in your hearts, Christ as he deserves, for he expects your mouths and tongues also, Rom. 10.9. If thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thy heart thou shalt be saved. Thus David was not satisfied with having the word of God hid in his heart, but faith, I will speak of thy testimonies before Kings, and will not be ashamed, Psalm 119.46, and (which makes the argument more forcible) the Kings of his time were Pagans, both strangers and open enemies to Religion.