A good one from Helmut Thielicke’s A Little Exercise for Young Theologians:
My plea is simply this: every theological idea which makes an impression on you must be regarded as a challenge to your faith. Do not assume as a matter of course that you believe whatever impresses you theologically and enlightens you intellectually. Otherwise suddenly you are believing no longer in Jesus Christ, but in Luther, or in one of your other theological teachers. (p. 31)
That one kind of slapped me in the face.