I believe that once we are saved, nothing can cause us to lose our salvation

Answer:

Sealing an object was done to guarantee that the contents were not tampered by another. For example, when Christ's tomb was sealed (Matthew 27:66) it meant that soft clay was put across the opening and an embossed seal was used to mark the clay. Any shifting of the stone would break the fragile seal demonstrating that the tomb was tampered with.

By declaring that we are sealed with the Holy Spirit is to declare that we have been marked as God's property. "Nevertheless the solid foundation of God stands, having this seal: "The Lord knows those who are His," and, "Let everyone who names the name of Christ depart from iniquity."" (II Timothy 2:19). The warning in Ephesians 4:30 is that those who belong to God should not be rebelling against God's authority as the Israelites did so long ago. "But they rebelled and grieved His Holy Spirit; so He turned Himself against them as an enemy, and He fought against them" (Isaiah 63:10). Even though the Israelites were chosen to be God's own possession, it did not imply that they could not rebel nor that God would not turn against them in their rebellion.

Ephesians 1:13-14 is an allusion to the same promise made to Israel under their covenant. "For you are a holy people to the LORD your God; the LORD your God has chosen you to be a people for Himself, a special treasure above all the peoples on the face of the earth. The LORD did not set His love on you nor choose you because you were more in number than any other people, for you were the least of all peoples; but because the LORD loves you, and because He would keep the oath which He swore to your fathers, the LORD has brought you out with a mighty hand, and redeemed you from the house of bondage, from the hand of Pharaoh king of Egypt. Therefore know that the LORD your God, He is God, the faithful God who keeps covenant and mercy for a thousand generations with those who love Him and keep His commandments; and He repays those who hate Him to their face, to destroy them. He will not be slack with him who hates Him; He will repay him to his face" (Deuteronomy 7:6-10). God had guaranteed to the Israelites that they would receive as an inheritance the land of Canaan which He had promised to their forefathers. His selection of the Israelites as a special treasure was a part of the keeping of that promise. In the same way, God has made a promise to Christians that they will receive an inheritance in heaven. The Holy Spirit is the guarantee of that inheritance because it was through work of the Holy Spirit in the inspiration of the apostles and prophets that we received word of that promise. In a sense, God is saying "You have my word upon it." Knowing that God always keeps His promises, His word is our firm assurance that the promise will be kept.

However, recall that though God gave Israel the land He promised, it wasn't without conditions. When Israel rebelled, they were removed from the land. Their removal did not mean God went back on His word. God kept His promise. It was Israel who did not keep their end of the covenant. In the same way, Ephesians 1:13-14 tells us that God has a promised inheritance awaiting and that no external force can prevent that promise from being fulfilled. Yet, it does not state that we, who have been promised the inheritance, cannot rebel and forfeit our rights to that inheritance. "Therefore, since a promise remains of entering His rest, let us fear lest any of you seem to have come short of it. For indeed the gospel was preached to us as well as to them; but the word which they heard did not profit them, not being mixed with faith in those who heard it. For we who have believed do enter that rest, as He has said: "So I swore in My wrath, 'They shall not enter My rest,'" although the works were finished from the foundation of the world. ... Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience" (Hebrews 4:1-3, 11).

The promise in John 10:27-29 is of a similar nature. Jesus promises to give his followers (his sheep) eternal life. In receiving that promise they will never perish. No one can prevent this from happening. "And I also say to you that you are Peter, and on this rock I will build My church, and the gates of Hades shall not prevail against it" (Matthew 16:18). The assumption being made is that the gift of eternal life is being granted immediately, but that is not stated in this verse. In fact, Ephesians 1:13-14 and Hebrews 4:1-11 make it clear that the promise is still in the future awaiting us. Nor is this promise implying that people will not choose to stop following Jesus, and when they stop, lose their right to that promise.

Romans 8:38-39 continues the same theme. Nothing in the physical or spiritual world can come between us and God. Yet, this does not mean that we cannot leave God. "Behold, the LORD'S hand is not shortened, that it cannot save; nor His ear heavy, that it cannot hear. But your iniquities have separated you from your God; and your sins have hidden His face from you, so that He will not hear" (Isaiah 59:1-2).

The idea that Christ's death cleanses the believer from all sins -- past, present, and future -- is not taught in the Bible. Instead, there is a clear warning that we can slip back into sin. "For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. Anyone who has rejected Moses' law dies without mercy on the testimony of two or three witnesses. Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace? For we know Him who said, "Vengeance is Mine, I will repay," says the Lord. And again, "The LORD will judge His people." It is a fearful thing to fall into the hands of the living God" (Hebrews 10:26-31). Notice that the person being discussed "received the knowledge of the truth" and was sanctified by the blood of the covenant. Clearly, such a person had received salvation but in his current state would not receive a heavenly reward.

Peter gives a similar warning about slipping back into the ways of the world: "For if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning. For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them. But it has happened to them according to the true proverb: "A dog returns to his own vomit," and, "a sow, having washed, to her wallowing in the mire"" (II Peter 2:20-22). Again, the people Peter had in mind had "escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ." These people had been saved but came to a state worse than before they learned the truth.

The writer of Hebrews also warns that some people can go so far as to be unrecoverable. "For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame" (Hebrews 6:4-6). Without a doubt, Christ died for our sins, but his death does not prevent us from sinning, nor will it cover our sins if we persist in sin.

"There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit" (Romans 8:1). The key to understanding this verse is found in the last half of the verse. There is now no condemnation for those who are in Christ Jesus, who do not walk according to the flesh. A person who is in Christ and who is not sinning currently has no condemnation. But this verse does not state or imply anything about a person's future condition. There is nothing here stating that a person cannot leave Christ or start walking once again in the ways of the world.

You even acknowledge the fact that people can and do sin. I John 1:10 states, "If we say that we have not sinned, we make Him a liar, and His word is not in us." Therefore, having been saved by Christ does not imply that our struggles against sin has ceased. It has only begun. I John 1:9 gives the remedy for a Christian who does sin: "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." It is a comfort to know that we can be forgiven, but it remains conditional. What happens to the person who will not confess his sins? What happens to the man who insists that God has to take him just the way he is, sin and all? I John 1:9 does not indicate that such a person will be saved despite his stubbornness.