FAQ for
The Mission (1986) More at IMDbPro »

It's his old armor from when he was a soldier & a slave trader. He's dragging it because of the guilt he feels over killing his brother Felipe. This penance is self-impose which is why, despite the other priest cutting the rope, he goes back & ties it to himself again. When the native man cuts the rope, Mendoza feels he has been forgiven by the people he used to capture for the slave trade.