How does 1 John 5:12 help you here? All that says is that one who has the Son has life, and one who doesn't, doesn't. There is no implication at all that it is possible to lose the Son once one has Him. As I recently posted, these verses, however, all say that once one is saved he will not become unsaved:
Eph 1:11-14 : 11 In him we were also chosen, having been predestined according to the plan of him who works out everything in conformity with the purpose of his will, 12 in order that we, who were the first to hope in Christ, might be for the praise of his glory. 13 And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, 14 who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession — to the praise of his glory.
Eph 4:30 : And do not grieve the Holy Spirit of God, with whom you were sealed for the day of redemption.
2 Cor 1:21-22 : 21 Now it is God who makes both us and you stand firm in Christ. He anointed us, 22 set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come.
I suppose only in the Apostolic faith is God's guarantee not a guarantee.
FK: "It is righteous to preach against backsliding."
Why? You are "already" going to heaven, no matter what you do. God has "already ordained it", so no matter what you do, good or bad, you are going to heaven...
Because the Bible says so:
Rom 6:1-4 : 1 What shall we say, then? Shall we go on sinning so that grace may increase? 2 By no means ! We died to sin; how can we live in it any longer? 3 Or don't you know that all of us who were baptized into Christ Jesus were baptized into his death? 4 We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life.
God's plan for all His children is to live a new life, not fall back into backsliding. We do not live our lives as Christians in order to earn enough points to get into Heaven. Regardless that we are already saved and have assurance, God still has great plans for all of us while on this earth. We recognize that and want to serve.
God's guarantee is to those who do not deny Christ by their actions. Those who remain faithful through obedience. Those who abide in Christ. Paul writes to a community that he presumes will remain in Christ. He is not speaking to the individual and guaranteeing them individually salvation. It is the community, the flock, that he speaks of. Those who remain within will be "saved".
And as you pointed out, just because one says "Lord, Lord" is not proof of your being "in Christ"
Regards