The quoted material below comes from my post, Perseverance of the Saints Part 12: Examining Passages Commonly Appealed to by the Advocates of Unconditional Eternal Security. The bold sections in between these quotes are further reflections and exegetical notes on the quoted material.
In Him, you also, after listening to the message of truth, the gospel of your salvation- having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory….Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption (Eph. 1:13, 14; 4:30).
Much has been made of the “sealing” of the Holy Spirit by defenders of unconditional eternal security. The “sealing” of the Holy Spirit is clearly conditional since we can “grieve” and eventually “insult” the Sprit of Grace, which constitutes total apostasy without remedy (Eph. 4:30, and Heb. 10:29). The Holy Spirit is received by faith (Gal. 3:2, 14) and can only seal us as we remain in Christ through faith. We are, in fact, sealed in Christ, by the Holy Spirit, as a direct result of faith (Eph. 1:13). The sealing of the Holy Spirit presupposes the possession of the Holy Spirit, and only believers can possess the Holy Spirit (Rom. 8:9). He is therefore the guarantee of an inheritance for believers and not unbelievers.
This is very important and must be understood in the context of our union with Christ. Through union with Christ we are “predestined” to adoption and an inheritance as children of God, being “joint heirs” with Christ through faith union with Him (Eph. 1:5, 10-11, cf. Rom. 8:17; Gal. 3:29; 4:7). This is what is in view when Paul speaks of being sealed “for the day of redemption”. The Spirit seals us in Christ through Whom alone we can reach our final destination. But we must not assume that this sealing is permanent or unbreakable (see below). The text does not say this at all.
There may be a parallel with circumcision which was also a “seal” for those under the old covenant (Rom. 4:11). We know that that seal was broken and guaranteed nothing when those who were circumcised broke the covenant and were cut off from the people of God (Rom. 2:25). The seal was conditioned on continued faith and obedience (2:26-29). The Holy Spirit marks us as children of the new covenant through faith in Christ, but if we abandon the faith then the Spirit of God no longer remains in us and we are no longer sealed in Christ (partakers of the covenant blessings that are found in Him alone- Eph. 1:3, 7, 10,11). Only those that continue in obedient faith remain sealed [by the indwelling Spirit] (Acts 5:32, Jn. 14:15-17; Rom. 8:5, 6, 9).
Romans 4:11 is devastating for any argument that insists that the use of the word “sealed” in Eph. 4:30 must have reference to an unbreakable and permanent seal; if that were the case then the “seal” (same Greek word) spoken of in Rom. 4:11 must also have been unbreakable and permanent. But that is clearly not the case. Instead, the sealing of circumcision was conditional and breakable through unbelief (see Rom. 11:20-23- circumcised Jews are broken off through unbelief, cf. Rom. 2:25-29). The sealing described in Rom. 4:11 is directly connected to and entirely dependent on continued “faith”. So it follows that as long as one believes, that person is sealed. But it does not follow that the sealing guarantees continued faithfulness or the inevitable attainment of a future destiny.
Notice that the sealing of the Holy Spirit is coupled with a warning not to “grieve” Him in Ephesians 4:30. This would seem to indicate that there is danger in grieving the Spirit who seals us and the reference to sealing may be for the primary purpose of reminding the Ephesians that to grieve the Spirit is to grieve the one who unites us to Christ. This makes the warning far more emphatic and cautions the believer to watch how he lives lest the sins which grieve Him lead to unbelief through which the seal is broken and the Spirit is finally “insulted.” The sealing of the Holy Spirit, therefore, applies only as long as we do not “grieve” (Eph. 4:30), and finally “insult” (Heb. 10:29) the “Spirit of Grace” through continued disobedience, culminating in outright apostasy.
This is very significant. While Calvinists and proponents of eternal security emphasize the “sealing” unto redemption, they tend to overlook the significance of the attached warning of grieving the Holy Spirit. Paul certainly attached the benefit of being sealed with the warning against grieving the Agent of sealing (the Holy Spirit) for a specific reason. It is significant that in the Old Testament God speaks of the Israelites who “grieved” His Holy Spirit in their rebellion (Isaiah 63:10). These Israelites were cut off from the promise of entering God’s rest. In their apostasy, they became God’s enemies,
“But they rebelled and grieved His Holy Spirit; therefore, He turned Himself to become their enemy, He fought against them.”
But of the future and faithful generation Isaiah speaks of the Spirit of the Lord guiding His people to the place of “rest” (vs. 14).
In the New Testament the “rest” in view for believers is the eternal rest that all believers will attain on the New Earth and in the New Jerusalem. The writer of Hebrews continually spoke of the eschatological promise for believers of eternal rest, and warned his believing audience not to miss out on this promised rest through hardening their hearts in unbelief, just as the Israelites did who rebelled against God in the wilderness (Heb. 2:1-4; 3:5-4:11; 6:4-8; 10:26-31; 11:13-16; 12:15-25, etc.).
So it seems very likely that Paul had these motifs in mind when writing Eph. 4:30. The Spirit seals believers for the “day of redemption” (the day of final eschatological rest for God’s people). It is for this reason that believers must be on guard so that they do not grieve the Holy Spirit who seals them to the point of effectively breaking that seal through unbelief, again making themselves enemies of God, and effectively missing the promised rest guaranteed to all believers on the condition of enduring faith. So when we understand Paul’s purpose in coupling the warning with the promise in Eph. 4:30, we see that the passage does much more to support conditional security than it does unconditional security.
[Therefore], there is no Biblical reason to see the sealing of the Holy Spirit as unconditional or irrevocable, while there are plenty of reasons to see it as conditioned on continued faith. Indeed, warnings against apostasy alone imply the conditionality of the seal.