But we ought always to thank God for you, brothers and sisters loved by the Lord, because God chose you as firstfruits to be saved through the sanctifying work of the Spirit and through belief in the truth.

--Parallel verses:
‭‭I Corinthians‬ ‭15‬:‭20‬-‭23‬
But Christ has indeed been raised from the dead, the firstfruits of those who have fallen asleep. For since death came through a man, the resurrection of the dead comes also through a man. For as in Adam all die, so in Christ all will be made alive. But each in turn: Christ, the firstfruits; then, when he comes, those who belong to him.

Firstfruits is an interesting concept, because it was required of man by God all the way back in the beginning (Genesis). Abel and his brother Cain were required to offer the firstfruits of their labor. Abel was a farmer and offered God the best of his crop and was excited in attitude and heart to give it to him. Cain was a livestock owner, and he did not offer God the best of his livestock, coupled with an attitude of thanksgiving, in a way that pleased the Lord. So Cain was jealous of Abel and killed him.

In a reversal of sorts, Jesus is giving God a firstfruits offering to rescue us — his life as a sacrifice once and for all to atone for our sins. And we who are in Christ will together with him offer up our bodies as a living sacrifice. See here … “Therefore, I urge you, brothers and sisters, in view of God’s mercy, to offer your bodies as a LIVING SACRIFICE, holy and pleasing to God—this is your true and proper worship.” (‭‭Romans‬ ‭12‬:‭1‬, emphasis mine)

Jesus is the firstfruits sacrifice — a perfect offering to God.  And we who are believers in him are also a sacrifice to God.

Paul was calling the Thessalonians a sort of firstfruits of their ministry to the Gentiles. Also, likely a firstfruit of the Church, which would expand to every nation.

We are perfected by the heart-sanctifying work of the Holy Spirit and by the living Word of God worked out in our lives — the Word of Truth.

Lord, perfect me in your Spirit and your truth, I pray. May my body be pleasing to you as an offering when I am raised up when I die, or when you come again. Thank you for sending Jesus as a firstfruits offering to redeem my sinful heart, which was heading for destruction. You have given me a new heart, a heart after you.