The word "first-fruits" is always in the plural, and designates “the first of a season´s agricultural products, offered to God; the first results of work etc.” (The Oxford Everyday Dictionary).
It is used many times in the Old Testament, especially with reference to a day to annually celebrate the first harvest, as the Lord commanded Moses: "Speak to the children of Israel, and say to them: 'When you come into the land which I give to you, and reap its harvest, then you shall bring a sheaf of the firstfruits of your harvest to the priest. He shall wave the sheaf before the LORD, to be accepted on your behalf; on the day after the Sabbath the priest shall wave it." (Leviticus 23:10-11). It must noted, then, that this had to be done on the first day of the week, our Sunday.
Three other sacrifices and offerings were to be made on that occasion, and in Numbers 28:26 to 31 further instructions were given about this ceremony, with more details about the offerings and sacrifices. We will not consider these details here.
Seven weeks later, therefore on another Sunday, He ordered that a new offering, of cereal, was to be made to Lord (Leviticus 23:15,16). This was known as the day of Pentecost (fifty days).
These two Sundays are prophetic, and their fulfilment was as follows:
1. The first Sunday, that of the first-fruits, was a symbol of the Sunday of the resurrection of the Lord Jesus: "But now Christ is risen from the dead, and has become the firstfruits of those who have fallen asleep." (1 Corinthians 15:20). He gave His life on the cross on the day of Passover (the date was fixed as the fifteenth day of the first Israelite calendar month, Nisan, and so it might occur on any day of the week). There was no fixed date for the feast of first-fruits, but it must only be celebrated on a Sunday at the start of the spring harvest, which was the first harvest of the year. On that occasion it happened on the Sunday after the week in which they eat the Passover, and it was on this day that the Lord rose again.
Christ being "the firstfruits," how can we explain the other resurrections mentioned in the Bible starting from the Old Testament? The answer is that they were different: they consisted of a simple return to natural life. The dead were as if sleeping (John 11:11), and all definitely died again later; the resurrection of the son of God was the first in which "this corruptible must put on incorruption, and this mortal must put on immortality" (1 Corinthians 15:53), i.e. thereafter the human body never dies.
While some people were resurrected, lived and then died again, only Christ has until now enjoyed permanent resurrection, and therefore it is said that He is "the firstfruits". But the term "the firstfruits of those who sleep" forcibly dictates that there will be more of the same series after the first One, which is the Prelude.
In 1 Corinthians 15:21 we have an explanation of the logic of divine justice: "For since by man came death, by man came also the resurrection of the dead". Only a human being could bring about the resurrection of the human dead, and this man needed to be free from sin so that his death could serve as a punishment for the sin of others who, so justified, might attain immortality.
In the following verse these two pioneers from whom consequences have arisen for humanity in general are identified: "as in Adam all die, even so in Christ all will be made alive". All mankind is condemned to death because of Adam, the first man, without hope of returning to life by themselves: mankind cannot produce one human person without sin. This is the most important reason for the son of God taking upon Himself the form of man: through the action of the Holy Spirit, Jesus was raised from a woman without man's interference. So He did not inherit the sin of Adam. In His perfect justice, compassion, mercy and love God himself paid the penalty of sinful man through the later death of his innocent Son.
All who are in Christ are made alive in a spiritual sense: “And you He made alive, who were dead in trespasses and sins” (Ephesians 2:1). They are not only made alive, but they immediately receive other great blessings such as the adoption as children, an elevated position, as well as future rewards, etc. With all the righteous from other ages, they will also take part in the physical "resurrection of the dead” of 1 Corinthians 15:21. Verse 23 gives a further detail: "But each one in his own order: Christ the firstfruits, afterward those who are Christ's at His coming.”
Here a Greek word tagma is used meaning a military procession, following a certain sequence. After Jesus Christ, it will be the turn of those born again by the Holy Spirit: they are the believers, the saints adopted as children of God and gathered in the church of Christ. In the same way as the Lord Jesus, they will rise from the dead when He comes to snatch His own from the wrath of God that will come upon the rebel and incredulous world, and the "great tribulation" of Israel. At this time they will join their brethren which are still alive (1 Thessalonians 4:17), all will be like Him (1 John 3: 2) and be taken to be with Him forever.
Finally, after the second coming of the Lord Jesus Christ with His angels and saints to destroy the enemies of Israel and establish his millennial Kingdom, it will be the time for the resurrection of the saints of the Old Testament and of the tribulation, who will be raised and also be present from the beginning of the Millennium. The "first resurrection" will then be complete (Revelation 20:5 and 6).
There will be a second resurrection. About six centuries before the coming of Christ, Daniel received the Lord's prophecy that "... many of those who sleep in the dust of the ground will awake, some to everlasting life, others to shame and everlasting contempt." (Daniel 12: 2). The dead since the beginning of mankind that did not participate in the first resurrection will attend before "a great white throne" to be judged according to their works, and those who do not have their names inscribed in the Book of Life will be cast into the "Lake of fire" in the terrible eternal punishment, called "the second death" (Revelation 20:11 to 15).
2. The second Sunday was a symbol of the day of Pentecost, seven weeks after the Sunday of the resurrection; the Holy Spirit came on the first church formed by the disciples of the Lord Jesus, gathered in Jerusalem on that day. Like the cereals offered by Jews on that day, this was the first tribute to God by Christ of redeemed lives after His death and resurrection. That same day there were thousands of conversions, giving a powerful beginning to Christianity in that world of Jewish rebels and idolatrous Gentiles.