Well, I guess you could blame the conversion of the Guarani on the oboe! (Ha!) You see, the Guarani weren't easily converted, as one of the iconic scenes from the beginning of the movie is of a crucified priest flowing over the Iguazu falls. Hmmm, originally not to fond of the Catholics, I guess.
It is Father Gabriel who finally piques the Guarani's interest by simply sitting in the jungle and playing his musical instrument. (Of course, first he had to prove his worth by climbing the expansive falls, mind you.) Although they are preparing to kill Father Gabriel, the Guarani are absolutely entranced by the music of the oboe that they've never seen or heard before. Father Gabriel reaches out to the tribe who eventually welcome him. Father Gabriel becomes their benefactor of both religion and education.
I would be amiss, however, if I didn't mention the very special conversion scene burned into my mind through music of the native language of the Guarani, but sounding distinctly like they are singing, "Love song, sing a love song, ..." It is through this medium that the director shows how the Jesuits, on their return journey, go about converting the entire tribe complete with both churches and schools.
Truly an inspirational song. Truly an inspirational movie.