Actually, they don't know of this doctrine specifically, but the "classism" is Biblical and pertains to them. It's referenced in the VINEYARD prophecy where Christ hires workers at the first through 11th hours, but the 3rd through the 11th-hour workers are not told in advance of what they will earn and Christ says, "whatever is fair". Thus this establishes two classes; i.e. those who know they will get the "penny" up front and those who are settling for less. This is the situation of two classes beginning in 1935 which would recognize the appearance of these "third-hour workers".
If those verses in Matt 20:1-16 teach a class system, if that was the case then their should be three classes.
The first group agreed to the wage of a denarius for the day.
The second group hired at the third hour, the man says "Whatever is just I will give you".
The third group hired at the eleventh hour he just says "You go to the vineyard".
They were all expected to do the same work, the first group agreed to the wage.
The other two groups to what ever the master decided.
At the end of the day the master has decided to give the same wage to all. For it is his right to do so.
The gift of God is eternal life, but we will be rewarded according to our faithfulness in the work He has called us to. He gives the gift of salvation freely to all who ask, no matter how late in their lives nor how sinful they have been.
God treats us better than we deserve, whether we are Christians or not, the reward of the kingdom is not of debt, a gift- and the gift is more than we hoped for.
Jesus is showing that God can give to us out of the abundance of His goodness, completely apart from what is deserved
. Living under grace is sort of a two edged sword - under grace, we can't come to God complaining, "hey, I don't deserve this!" because God will reply, "So, do you really want Me to give you what you deserve?"