It's the parable found in Matthew 20:1-16; the Master of a house going out to hire workers for His vineyard. Summed up, He hires some men in the morning (presumably 6am) under the promise of a full day's wages (one denarius), then hires more at 9am, 12pm, 3pm, and 5pm (third hour, sixth hour, ninth hour, and eleventh), all under the promise of a denarius. He has them paid last to first, as in those hired at 5pm to be paid first, then those at 3pm, then those at noon, and so on. The first-hired, last-paid get upset, saying they deserve more. The owner's response is that it's all generosity, and He can distribute His money as He wishes.
In this book, it's described as a grace and that, if paid according to what we deserve, we'd all get Hell. Instead, it's moreover gifts that God gives us rather than wages; that the grace of it is that it's not about counting, because grace doesn't work by measurement. And that's true, I will not deny that. It's a great example of grace in that sense.
Reading over that parable again, I've realized something I'd care to add to this interpretation.
Firstly, a lifelong Christian will get Heaven. A lifelong Christian who turns away in his final hour will get Hell. Likewise, a lifelong heathen will get Hell. Inasmuch, a lifelong heathen who repents in his final hour will receive Heaven. And with Heaven comes all the glories that God bestows upon His beloved.
The thief on the cross beside Christ, despite asking Christ to remember him as he was dying, was given -that day- the same amount of Heaven as, say, the most notable reformer, the widest-reaching evangelist, the most faithful disciple, or the greatest of apostles. They all received the same Heaven, the same God, the same blood of Christ covering them. This is grace, but it's also fair in the sense that if any of these people had disowned Christ, even a second before their final breath, they would have received the same amount of Hell, despair, and absence from Christ.
Secondly, this is reminiscent of the mentality of the first son in the story of the prodigal; when the younger son demands his inheritance, it's made clear that the older gets his share as well, at the same time. Yet, when the younger returns after having squandered everything, the older son is irate when the Father throws a party, has the prize calf butchered for a feast, and the younger son's return treated as a monumentally joyous occasion. His case is that the Father had never thrown a party for him, saying, “Look, these many years I have served you, and I never disobeyed your command, yet you never gave me a young goat, that I might celebrate with my friends. But when this son of yours came, who has devoured your property with prostitutes, you killed the fattened calf for him!” (notice how he doesn't even claim his brother, except as “this son of yours”)
It doesn't seem fair, but it is. The Father's response was “Son, you are always with me, and all that is mine is yours. It was fitting to celebrate and be glad, for this your brother was dead, and is alive; he was lost, and is found.”
In both senses, the fault lies in the ones receiving; it's not up to them (this is you and I) to bargain with God, nor to look at what others are getting. We should focus on how He's blessed us, not how He's blessed another person for the same (or less). Our walk is not with them, it is with God. Our walk doesn't concern them, nor does theirs concern us. Both concern God, and it's between the two.
To paraphrase the way one of my favorite singers put it in her book, titled “What Is The Point?,” God doesn't measure the size, timeframe, or impact of our ministry. He measures the heart of it. If we help lead one soul or one thousand souls to Him, He does not favor one over the other; He watches the heart we put into the one or the one thousand.