Firstfruits

As good a summary as you will read on firstfruits in the Bible:

In the Old Testament the firstfruits offering was not only the beginning but also the best part of the whole harvest. As a gift that represented the totality, it was both the first and the finest portion of everything the Promised Land produced. Accordingly, when Paul tells us that God has given us the firstfruits of the Spirit, he is saying that we have received not only the beginning, but also a taste of the best part of the fullness God has planned for the future.

This language shifts the picture from God as recipient to God as giver. Israel's understanding of firstfruits was always as something given to God. Yet now it is God who gives to us the firstfruits...the God of the universe gives offerings to the likes of us and that choice offering is himself...

God's new creation began with Jesus' resurrection - which explains why he is called the 'firstfruits' from the dead (1 Cor. 15:20, 23) - and continues with his Spirit... those who are in Christ and have received the Spirit are now themselves called 'the firstfruits,' having been saved and sanctified by the Spirit (2 Thess. 2:13; James 1:18; Rev. 14:4).

(p. 102, Kelly M. Kapic, with Justin L. Borger, God So Loved, He Gave: Entering the Movement of Divine Generosity)