Growing up with a heap of reggae records and a tendency to overanalyze lyrics at 2 a.m., I always traced 'one love' back to a mix of older spiritual ideas and the music scene that distilled them. The phrase itself rides on a long human tradition — think Christian 'agape' love, Buddhist 'metta', and general calls for unity you find in many religions and philosophies. But as a specific catchy phrase, it really came into global circulation through Jamaican culture and music in the 20th century.
In musical history, the Wailers had an early version of 'One Love' in the mid-1960s, and a later, hugely influential take was released by Bob Marley and The Wailers as the medley 'One Love/People Get Ready' in 1977, which actually weaves in elements of Curtis Mayfield's 'People Get Ready.' That recording, coupled with Rastafarian ideas of unity and Marcus Garvey’s Pan-African rhetoric circulating in Jamaica, cemented 'one love' as both a slogan and a worldview. To me it's always felt equal parts spiritual imperative, political solidarity, and pop-culture hook — the sort of phrase you can sing at a festival or carry into a protest march.