Some secular slogans:
Good without God
Seeing is believing.
Assume nothing. Question everything.
Extraordinary claims require extraordinary evidence.
We are stardust.
"We are made of star stuff." —Carl Sagan
"Those who can make you believe absurdities can make you commit atrocities." —Voltaire
"Those who cannot remember the past are condemned to repeat it." —George Santayana

My own:
Wishful thinking is not critical thinking.
Free-thinking is not group-thinking.
The best argument against the Bible is the Bible itself.
Literalism of literature is illiterate.
Never forget the Dark Ages.