Understanding the Guarantee of the Holy Spirit
To my knowledge, this concept appears three times in Scripture, all within the body of Paul's writings: II Corinthians 1:22, II Corinthians 5:5, and Ephesians 1:13-14. "Guarantee" is the ESV rendering of the Greek arrabōn. Other translations use "earnest" (KJV), "deposit" (NIV), "down payment" (CSB), or "pledge" (NASB).
I think it's helpful to consider the nuances of all these words in order to figure out what Paul is saying here. An arrabōn is more than a promise. Instead, it is a tangible symbol of good faith. If I were to rent an apartment, my mere promise wouldn't be enough to convince the landlord that I wouldn't trash the place. I'd have to put down a deposit, and if I left the apartment knee-deep in filth, I'd lose the deposit. That knowledge would presumably keep me from living like a pig, to the landlord's benefit.
Similarly, when I bought my house, the lender wasn't satisfied by my avowal that I would honor the terms of the loan. They required a down payment from me. Now, if I lose my job and can no longer manage the mortgage, I can't just mail the lender the keys and go skipping off into the sunset. I will lose tens of thousands of dollars if I try. My down payment assures the lender that I will follow through on my promise, so they can lend me money without fearing that I will back out.
Much the same logic applies to the guarantee of the Spirit. God has made some spectacular promises to His people. In II Corinthians 1:21, it is that we have been established in Christ together with the apostles. In II Corinthians 5:1, we are promised a house not made with hands, eternal in the heavens. In Ephesians 3:14, we are promised an inheritance in heaven. Paul uses the guarantee of the Spirit to confirm these three promises.
The guarantee of the Spirit, then, must fill an evidentiary function. It must in some way confirm God's good faith. If we live for God, but He doesn't fulfill His promises and receive us into heaven, we are (to borrow a phrase from I Corinthians 15) of all men most to be pitied. The guarantee of the Spirit helps us to be sure that God isn't playing us for chumps.
A Personal Indwelling?
Many Christians believe that the guarantee of the Spirit refers to a personal indwelling of the Spirit imparted at the moment of salvation. However, that simply doesn't line up with the way Paul is using the phrase. Most brethren who believe in this personal indwelling do not think that they have been given any miraculous gifts along with it. The Spirit within them cannot be seen or felt. At most, He affects their conduct, but in a way that is indistinguishable from ordinary human impulse.
If that's all the Spirit is doing, He is in no way a guarantee of anything. He offers no proof, either to that Christian or to anyone else, that an inheritance in heaven is waiting for them. Such a "guarantee" is equivalent to a 0% down payment, and we saw during the Great Recession how well guarantees like that worked!
A Sign of Good Faith
Once we understand that the context requires the guarantee of the Holy Spirit to be a tangible sign of good faith, we don't have much trouble figuring out what Paul is talking about. In the first century, this requirement was fulfilled by the miraculous spiritual gifts. Need more assurance that God will give you eternal life? Look at this guy who can speak 20 different foreign languages that he never studied. Look at this other guy who can heal the sick instantaneously and even raise the dead. No subjective assurance in the heart can compare with evidence like that!
Today, of course, the miraculous spiritual gifts have departed. However, we still possess the work of the Spirit in the form of the word. Now the Bible is the guarantee of our inheritance. It doesn't merely repeat the promises of God. It confirms that those promises can be believed.
First and foremost, the Scriptures present a case for the resurrection of the Lord that even the most committed skeptics can't debunk, and from the resurrection, every other element of our faith follows. We find similar evidence in the pattern of fulfilled prophecy. Without divine help, David could not possibly have known that Jesus' enemies would gather around Him, mock Him for trusting in God, pierce His hands and His feet, and gamble for His clothes. However, David predicted those things 1000 years before the Lord drew His first breath, and there are dozens of similar examples. The work of the Spirit thus provides us with assurance that we can trust God.
Here, as always, the Bible calls us to a reasoned faith. We're not asked to believe because of a guarantee that guarantees nothing. We are asked to believe in a future that we cannot see because of Spirit-provided evidence that we can see. This is how the guarantee of the Spirit works: from the hearts of the gifted to our hearts through the word.