In Luther's day -- and throughout all of church history -- folks have sought a solid assurance of their salvation. In Luther's time, one of the false means sought was by trusting in letters of pardon from the Pope. That's what thesis 52 is addressing:
52. The assurance of salvation by letters of pardon is vain, even though the commissary, nay, even though the pope himself, were to stake his soul upon it.
The reality is that we are assured of trusting in the word of God. Most particularly, we trust in the words spoken to us in the Lord's Supper: "the Body of Christ, given for you," "the Blood of Christ, shed for you."
For you: that's where we stake our salvation. Knowing that on that Friday afternoon when our Savior died, it was for you, for me, for each Christian. Was it for the sins of the whole world? Of course. But we hear the words "for you," to assure us that when we despair of our sins, and when we imagine that our sins are different, that we have come one time too many for forgiveness, that even when our sins are grievous, our Savior's love and mercy is greater.