God keeps us saved for as long as we want to be saved.
I'm a believer of free will.
I don't believe God keeps us saved if we wish to walk away from Him. If He did it just means that we're a bunch of robots which He controls. I don't believe God wants people to love Him out of force, the bible teaches me that He wants us to love Him freely.
The sealing of the Holy Spirit is true. We are sealed as an earnest. As a down payment, as assurance of an intent to pay. But we must make the payment to receive what was promised in the end. Much like making a down payment for a car, if we don't make the payments every month, in the end the car will not be ours.
The Holy Spirit dwells in us.
1 Corinthians 6:19-20
Our body is a temple of the Holy Spirit which is within us. We have been bought with a price, therefore we are to glorify God. And what if I want to stop glorifying God? Am I not allowed to do so by my free will?
2 Corinthians 7:1
Having these promises, we are to cleanse ourselves from all defilement, perfecting holiness in the fear of God.
Yes. Our salvation is up to us to continue in the word of God.
The sealing is a down payment guaranteeing us what is to come which is far better, our entry into heaven, IF we continue in our salvation, or sanctification.
The bible plainly shows this in many verses.
2 Peter 3:17
17You therefore, beloved, knowing this beforehand, be on your guard so that you are not carried away by the error of unprincipled men and fall from your own steadfastness,
1 John 2:24
24As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father.
Romans 11:22
22Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off.