Ether 12:17

And it was by faith that the three disciples obtained a promise that they should not taste of death; and they obtained not the promise until after their faith.

It was by faith that the three disciples got a promise that they would not die. They did not get the promise until after they had faith.

It was through faith that the three disciples received the promise that they wouldn’t die. They didn’t receive that promise until after they had faith.