Christ is in you, I think, and I believe that he is in me. That does not ensure my salvation.
Hebrews 10:16 This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them; 17 And their sins and iniquities will I remember no more. 18 Now where remission of these is, there is no more offering for sin. 19 Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus, 20 By a new and living way, which he hath consecrated for us, through the veil, that is to say, his flesh; 21 And having an high priest over the house of God; 22 Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
That's just it, if Christ is in you, then he will never leave you or forsake you. He will not cast you out OR lose you. You are his when you receive him as Lord and Savior and the Holy Spirit indwells you as the "earnest of your inheritance". If that doesn't "ensure" your salvation, what else could?
So if you believe that Christ is in you what happens upon your death? Does He just abandon you at that point then?