In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory.
The Holy Spirit is a pledge, a guarantee, a "down payment" for our inheritance. We are sealed by the Holy Spirit at salvation and permanently indwelt by the Holy Spirit. This is akin to earnest money. Nowhere in the New Testament is the removal of the Holy Spirit provided for or described. If we could lose our salvation, then the Holy Spirit would have to accompany us to Hell and remain there with us. One of the reasons God gives us the Holy Spirit in this dispensation is to demonstrate that we are His and that nothing can ever change that. His Holy Spirit is the guarantee of that. Does God break His own guarantee? Not likely.