In 1 Cor 15 the First Resurrection is compared to the planting and harvesting of a crop. Harvesting has 3 parts:
1) It began with the gathering of the firstfruits which were offered as a sacrifice of thanksgiving to God.
2) It proceeded with the general harvest. But not all was taken in this harvest.
3) Some of the crop was left in the field to be gathered by the poor and the needy. This was called the gleanings (Leviticus 19:9-10).
There are also 3 parts to the First Resurrection.
1) Using this imagery, the Bible presents the resurrection of Jesus as the firstfruits of the resurrection of the righteous.
2) The gathering of the Church Age saints, living and dead, at the appearing of the Lord (the Rapture) is thus the general harvest stage of the resurrection of the righteous (Jn 14:1-3 & 1 Thess 4:13-18). 2
3) But there is a third and final stage to this resurrection of the righteous. It is the gleanings, and it occurs at the end of the Tribulation when the Lord’s Second Coming takes place. At that time the Tribulation martyrs (Rev 20:4) will be resurrected.