What the Bible says about Guarantee
(From Forerunner Commentary)

Romans 8:23 The apostle uses the word "firstfruits" in relation to redemption, and he employs "Spirit" in the same general sense as "guarantee" of the Spirit as in Ephesians 1:14. Firstfruits here literally means "a beginning," indicating a start has been made and more will follow. John W. Ritenbaugh Our Uniqueness and Time Related Topics: A Beginning | First Fruits | Guarantee of Our Inheritance | Guarantee of the Holy Spirit | Our Uniqueness | Our Uniqueness and Time | Redemption