The New Testament word for “guarantee” (Greek arrabon) has a peculiar meaning. I’m not sure what you think of when someone gives you a guarantee, but it is usually a verbal promise of some sort.
In the biblical world, a guarantee would usually be accompanied by a pledge, or a token, portion of the whole of the item promised. In the OT, a pledge was given by Judah (in a really *cough* complicated story in Genesis 38:17-20). Judah provided his seal and staff to Tamar as a promise that he’d send a young goat from his flock after he returned home.
The guarantee was supported by a tangible pledge, an item pointing towards the whole.
This idea is used by Paul in Ephesians 1. In speaking of the Ephesian’s hope for salvation, Paul said,
“When you believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession – to the praise of his glory.” (Eph. 1:13-14)
A common anxiety I hear as a pastor is, “How do I know that I belong to God?” Paul’s answer is that the Spirit is given to us in order to point towards God’s renewed world, that we get to both to anticipate later and to participate in today.