In this poem, Donne is trying to convey the idea that he and his love were essentially created when they started to love one another. Before that, they were nothing, or at least they were not anything like the real people they are now.
In the first stanza, Donne is trying to show what he and his love were like before they loved. He compares them to little children or to country bumpkins. Then he uses the line you cite. In that line, he is saying that their previous lives were like being unconscious or asleep. He says that they were like people who were "snorting" -- meaning snoring.
According to eNotes' summary and analysis of this poem, the line refers to a legend of persecuted Christians who were walled up in a cave by Romans in Syria and left to die. Instead, they miraculously slept rather than dying. The poet is saying that he and his love were like these people -- not dead, but not really alive and conscious either.