Traditionalists sometimes use the phrase, the Faith Once Delivered. The phrase alludes to a passage in Jude 1.3 (King James Version): "it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints."
The phrase makes for an intriguing TLA (three-letter abbreviation): FOD. It's intriguing because on an aircraft carrier, FOD stands for Foreign Object Debris — things like stray tools, nuts and bolts, etc.
On a carrier flight deck, the stuff that comprises FOD has its proper uses, at the right times and with appropriate safety precautions in place. But FOD is very dangerous if it’s carelessly left lying about on the deck when it's not the right time. It can easily get sucked into aircraft engine intakes, causing major damage and possibly even killing people. (That’s why flight-deck crews take great pains to keep the deck clear of FOD.)
It’s not hard to see parallels between FOD on aircraft carriers and FOD in the church. The Faith Once Delivered has its proper uses, at the right times and with appropriate safety precautions in place. But it can be pretty dangerous otherwise.