I got to thinking about that little prayer: "Now I lay me down to sleep."
I did a some research, and it seems that the prayer was included in a 18th Century New England Primer, which became the foundation for education prior to 1790. The New England Primer was published between 1687 and 1690 and was based on the The Protestant Tutor (published in England).
I pray the Lord my soul to keep
if I should die before I wake
I pray the Lord my soul to take.
The underlying theology of the prayer reveals that the Evangelical-assurance-of-heaven is a relatively new theology. The question posed to many Catholics today by Evangelicals is this: "If you die tonight, do you know that you'll go to heaven?"
Most Catholics would say (rightly so) that they entrust their souls to the mercy of God. Jesus is the guardian of the soul. I entrust myself to His mercy. But no, I don't know I'll be saved in a take-it-to-the-bank kind of way. Who does? I have to keep going and growing. I have to run the race well. And even then, I plan on hitting my knees when I enter eternity. I will cry out, "Lord, have mercy." Kyrie Eleison.
"But don't you want to know for sure" the Evangelical replies.
Here's the thing, Protestants in the 18th Century didn't even believe in those kind of absolute assurances. If they did, the prayer wouldn't make any sense, would it? The prayer wouldn't have been written and it certainly wouldn't have been included in a primer for students. Instead, there would have been a prayer that followed more closely the Evangelical prayer of salvation. And it wouldn't have to be prayed every night. Interesting.
Truth doesn't change. What prayer is your best bet? Kyrie Eleison. And that prayer is timeless and trustworthy.