It reads in Luke 10:2 (parallel is Matt 9:37-38):
Therefore said he unto them, The harvest truly is great, but the labourers are few: pray ye therefore the Lord of the harvest, that he would send forth labourers into his harvest.
I've encountered people who interpreted this command from Jesus as applying to all people at all times. It appears they understand that Christians are the laborers and the field is the world at large; therefore, Christians ought to continue to pray for and (presumably) be laborers. However, I don't think there's evidence Jesus intended this command to be for future Christians.
There's another harvest in Matt. 13 where the harvesters are clearly angels, and in Rev. 15 those that harvested were an angel and Christ. It doesn't make sense to pray for angels, and elsewhere it is implied the collective of Christians should ask for Christ's return (Rev 22:17), but that's a prayer in a different context.
In the epistles, Paul talks of being a laborer/farmer in II Tim 2, but it's not in an evangelical context. In I Cor 3:6-10, Paul uses the farming metaphor explicitly, it appears evangelical in nature, but he covers the whole process (planting, watering, etc.), but not the act of harvesting. This convinces me he's not reiterating Jesus' command here like he's done elsewhere, plus the context is completely different.
In the case of Luke 10:2, context implies that the harvest Jesus is speaking about are specifically the Jewish people and harvesters are those who Jesus personally sent out at the time he gave the command. John 4 might play into this some way, but I've not quite made the connection.
Have I missed something? Should I think Christians are commanded to pray for laborers today?