Held (assumed, proposed, believed etc.) for a long time(adjective)
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Use "long-held" in a sentence
"He refers to a long-held idea in esoteric circles of a mystical school of Christianity that was founded by John the Evangelist, based on the secret teachings he had been given by Jesus."
"Eisler, a former CIA agent and technology lawyer who gave up the profession to write thrillers, instead chose to break a long-held taboo: self-publishing."
"CHICAGO Reuters - The surprise drop in U.S. corn feed demand this summer may be only the beginning of a year-long reduction as chicken and hog producers break a long-held aversion to blending more wheat into rations."