Cousin Jodi has asked me to write a post on "remaining in Christ"--I'll try to oblige.
Many of us grew up as Christians in a broadly evangelical context--which essentially means Baptist at least theologically, and probably also culturally too.
So, for those of us who did, remaining in Christ might even be a foreign phrase. I mean, I prayed the prayer, what else do you want to know? The other reality is that for a certain group within evangelical Christianity, there is another phrase that comes into play: "Once saved, always saved."
Evangelical Christianity can be divided rather neatly into the more Calvinist camp of the once saved always saved variety and the more Arminian camp, which, to the Calvinists' horror, believes one can lose one's salvation, thus drawing God's sovereignty into question.
For my two cents--prepare to be shocked those of you who still live in the Baptistic culture (and even more shocked if you live in the Reformed culture)--the whole Calvinist/Arminian debate is a colossal waste of time! The catholic perspective is that God is sovereign and we have free will (I'll save that discussion for another time) and our salvation is connected to both of those ideas. Essentially, we are saved by the grace of God, not anything that we have done--be it belief, faith, baptism or the "sinners prayer".
But faith is a necessary part of that process called salvation, as is baptism, and for many a prayer is also part of the process.
Of course, defining salvation is important to this discussion, so let us quickly define salvation as "being in Christ". Of course, salvation is not just a once for all event, but St. Paul and others continually remind us to keep working out our salvation, in fear and trembling even. We were saved, are being saved, and will finally be saved.
So, if salvation is a continual process, then we ought to be concerned about continuing to be in the process--or as we might put it, remaining in Christ. We are incorporated into Christ in our baptism. That means we become a part of His body. Remaining in Christ is essentially remaining a part of His body, the Church.
How do we make sure to do that? Primarily, that means we participate in the corporate life of the Church in worship; in Word and Sacrament. When we cut ourselves off from the means of our feeding and strengthening, then it is us who dies, not Christ's Church.
Our salvation lies in Christ, and He has ordained that to be a part of Him is to be a part of His Church. They are one and the same.
Noah had faith, yes. Noah put that faith into practice in building the ark. But this did not save Noah. Noah could not just stand around in the rain and go on and on about his faith. That would have got him nowhere but drowned. Noah needed to put his faith in action and get himself into that ark. That's what saved Noah--by being in the ark. So too, we have to come, over and over again, to the ark of Christ. We need to leave the floodwaters behind and enter into the ark--to go into the Holy of Holies where the Ark of the Covenant is. We enter by the shed blood of Christ. We enter after confessing our sins--our dealings out there in the rain and the floods where there is death--and begging for God's forgiveness. We participate in the worship of the Church, hearing the Word read and preached and participating in the visible Word of the Sacrament of the Body and Blood of Christ.
This is (barring unrepentant sin) how we remain in Christ and where we find our assurance. It is a continual life in Christ, always repenting and confessing, always giving ourselvers, body and soul, to God, always reaching for the "prize of the upward call of God in Christ Jesus."
Hope that helps, Cous'--though none too eloquent, nor detailed. I'll do my best to answer specific questions!