Hi Gt,
Many who believed in Jesus turned away, we can turn back to our old life. We are sealed by the Spirit as long as we walk in the Spirit and worship God in Spirit and in truth and obey God. But if we turn back again to our old fleshly ways and put ourselves before God, then we are the ones who make the choice to put our lives before the will of God, and their are many who have done this, I know a couple who met in church, they have been together a long while, and they are now both atheists, are they eternally saved?
I hope that they will be, but we can't say that they are, I hope that they will turn to God again. We are just flesh, I pray to God always to keep me and to give me strength to overcome Satan every day, because I know that I can't do it without Christ and without the power of Gods Spirit in my heart. And I thank our Lord Jesus for showing us the way.
Many who profess to be saved love their flesh. You will know those who are truly being saved because they will be doers of the word and living by the will of God. If we're not, then we are just giving lip service and we are false witnesses.