When people say things like that, I like to ask them which words Jesus wrote with His own hand. If we can accept the testimony of Matthew, Mark, Luke, & John, why should we not accept the testimony of Paul?

At any rate, there's always the opportunity to quote more words from Jesus. Something like Matt. 10:32-36: "Every one therefore who shall confess me before men, him will I also confess before my Father who is in heaven. But whosoever shall deny me before men, him will I also deny before my Father who is in heaven. Think not that I came to send peace on the earth: I came not to send peace, but a sword. For I came to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law: and a man's foes shall be they of his own household."

Kyle

I tell you, this man went down to his house justified.