Rev 14:1 And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father's name written in their foreheads. 2 And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps: 3 And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth. 4 These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb whithersoever he goeth. These were redeemed from among men, being the firstfruits unto God and to the Lamb. 5 And in their mouth was found no guile: for they are without fault before the throne of God.
From Gill's Exposition of the Entire Bible Revelation 14 Gill's Exposition of the Entire Bible
" ... being the firstfruits unto God, and to the Lamb; in allusion to the firstfruits under the law, which represented and sanctified the lump, and showed that harvest was coming; so these persons are called the firstfruits to God, and to the Lamb, being called by grace, and consecrated to their worship and service, with reference to the harvest of souls, or that large number of them which will be gathered in during the spiritual reign of Christ, which these persons will be at the beginning of; and as those who are first called and converted in a country or nation are said to be the firstfruits of it, Romans 16:5; so these being the first, in the period of time to which respect is had, bear this name; and as the converted Jews received the firstfruits of the Spirit, on the day of Pentecost, and at other times, so these will receive the firstfruits of the far greater pouring forth of the Spirit in the latter day, which will begin, and usher in the kingdom of Christ; see Romans 8:23."
.