'tis worth my sufferings to gain so true a knowledge both of you and of your gods by whom you swear.
https://enlightenmens.lmc.gatech.edu/items/show/443
Oroonoko sees the suffering of his current position as an opportunity to learn the truth about the Christians. The relationship between suffering and truth has been established for a very long time. In Robinson Crusoe, the first time he was able to seriously think about what he has done and what he will do, was when he was confronted by the ocean, and was suffering on the boat from the barrage of the wind and waves. Life is a tragedy and seeing it without filters is painful and difficult, but it is the only way to find truth. This idea can be traced back to the biblical story of the garden of Eden, where Adam and Eve learned about truth, but had to venture into the world with the suffering in it. When Oroonoko says he knows the truth about "the gods by whom [they] swear," he is saying their lies and tricks will send them to hell, while his truth and honor will bless him and the fellow slaves in "the next world". This remark is unchallenged because the Christians there knows it is true, and that they do not live according to the principles they swear.