他知道，如果救援不能及早赶到，他们必死无疑。

He knew that if help did not arrive soon they would surely die.

《牛津词典》

要是他们的身份被敌人发现，那将必死无疑。

If their identity had been discovered it would have meant sure death.

youdao

活着的人，知道必死。死了的人，毫无所知。也不在得赏赐，他们的名无人记念。

For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten.

youdao

When people have these near brushes with death, it's easy to believe that the fact of their mortality is more vivid.

当人们拥有这种濒死体验，很容易相信他们对必死的感受会更深刻。

耶鲁公开课 - 死亡课程节选