Origin and history of tenacle

"principle, opinion, or dogma maintained as true by a person, sect, school, etc.," properly "a thing held (to be true)," early 15c., from Latin tenet "he holds," third person singular present indicative of tenere "to hold, grasp, keep, have possession, maintain," also "reach, gain, acquire, obtain; hold back, repress, restrain;" figuratively "hold in mind, take in, understand" (from PIE root *ten- "to stretch"). The connecting notion between "stretch" and "hold" is "cause to maintain."

The modern sense is probably because tenet was used in Medieval Latin to introduce a statement of doctrine.