Paul’s letter to the Ephesians talks about the Holy Spirit acting as a guarantee. He writes (Ephesians 1:8-14, NIV):
With all wisdom and understanding, he made known to us the mystery of his will according to his good pleasure, which he purposed in Christ, to be put into effect when the times reach their fulfilment—to bring unity to all things in heaven and on earth under Christ.
In him we were also chosen, having been predestined according to the plan of him who works out everything in conformity with the purpose of his will, in order that we, who were the first to put our hope in Christ, might be for the praise of his glory. And you also were included in Christ when you heard the message of truth, the gospel of your salvation. 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.
The passages starts by indicating that God took the first step in our salvation: he made Jesus known to us. Theologically, we might call this prevenient grace – it is the grace that allows sinners to reach out and accept Jesus.
Then, in addition to receiving this grace or initiative from God, we learn we were also chosen. The terminology comes right from the Old Testament.
Craig S. Keener in the NIV Cultural Backgrounds Study Bible writes: “Despite the ethnically mixed character of the church in Ephesus (see note on v. 3), Paul applies to the church significant language that the OT applied to Israel.” That could suggest a corporate view of election: those who were deemed to be “in Israel” were saved, but not every Israelite. In the same way, Ephesians 1 could suggest that those who are “in Christ” are saved as a collective. Paul says that his recipients who heard the gospel (and converted) were “also were included in Christ”.
Meanwhile, 1 Peter 1 explains how we can understand the idea of individual election. The apostle says that people are “chosen according to the foreknowledge” – i.e. God predestines those he knows will choose him. So God makes sure everything ties up, that there are no unwanted loose ends, but yet also ensures that people have the free choice to follow him (or reject him). He did not choose to built humans as drones but as thinking people, in his image.
Now we come to the part where the Holy Spirit is described as a seal. That’s a significant word because baptism is also considered to be a seal, as is circumcision under Judaism.
In Biblical times, a seal was used with wax to effectively sign or authenticate a document. It “indicated ownership and protection”, according to Te-Li Lau in the Biblical Theology Study Bible. Lau stresses that the seal “does not refer to some second blessing or subsequent action”, but note two things it offers. Firstly, it marks followers of Jesus as God’s people. Secondly, it preserves them until they receive their final redemption.
Of the first point, we know Galatians 5 that the Holy Spirit has fruits that affect our behaviour and move us away from “acts of the flesh” such as envy and hatred. Galatians does not say that humans won’t do any of the acts of the flesh if they are saved. Paul, instead, urges the Galatians to “keep in step with the Spirit”. But it helps change our attitudes and what we do.
On the second point, the Holy Spirit helps protect us in our salvation. This is not the same as the claim of “once saved, always saved”. That ignores the teaching of Hebrews with its warning passages about people themselves choosing to leave the faith. But there are good Biblical grounds for saying that God protects our membership of his covenant from the actions of others. In Romans 8:38-39 (NIV), Paul writes:
For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.
So the Holy Spirit is a protective force on our faith.