Summary: For a Christian to live "ln Christ" is the central doctrine of the New Testament. Except for limited portions that focus on the start of the Christian life by being born again, most of the New Testament is addressed to and can only be understood by Christians living "ln Christ".
Can I live in Christ permanently? - only in Christ can I bear fruit
The following three verses from John chapter 15 state three of the most central truths of Christianity (1) God's purpose for the Christian (2) The only way God's purpose can be fulfilled, and (3) The reason why many a Christian's life remains unfulfilled:
John 15:4 (Jesus speaking) Dwell in Me, and I will dwell in you. Live in Me, and I will live in you - Let me live in you TLBI. Just as no branch can bear fruit of itself without abiding in (being vitally united to) the vine, neither can you bear fruit unless you abide in Me.
5 I am the Vine; you are the branches. Whoever lives in Me and I in him bears much (abundant) fruit. However , apart from Me [cut off from vital union with Me] you can do nothing.
7 lf you live in Me [abide vitally united to Me] and My words remain in you and continue to Iive in your hearts, ask whatever you will, and it shall be done for you. AMP
God's primary purpose for the Christian? To bear much fruit. The fruit of the vine contains the seeds of the vine and are attractive to eat so that the seeds may be dispersed. And what exactly then is the fruit of the spiritual vine (Jesus) and His branches (Christians)?
Luke 6:44-45 For every tree is known by its own fruit...A good man out of the good treasure of his heart brings forth good...For out of the abundance of the heart his mouth speaks. (And what does the mouth speak? Words, they're his fruit)
The fruit of a Christian are his words. When a Christian's words are from his heart and Christ's words dwell there, and his words are anointed by God's Spirit because he is living in Christ, then those words are living fruit and contain the seeds of Christ. These seeds are ready to be planted into the heart of another when they hear with their heart the words spoken by the fruit bearing Christian and thus eat the living fruit that contain Christ's seed.
Clearly a Christian cannot bear living fruit, or speak living words, except Christ's words remain in his heart and he remains in Christ, that is, under the immediate control of Christ's Spirit. And so God's primary purpose for a Christian, to bear fruit and this includes the fruit of effective prayers, can only be fulfilled when that Christian has learned to live in Christ and for God's words to live and remain in him. lf a Christian chooses not to live in Christ - and the above three verses from John chapter 15 make it clear that it is our choice - then that Christian's life cannot be anything but barren, fruitless and unfulfilled, because he has chosen to live apart from Christ and not as a Christ-one.
For a Christian to live "ln Christ" is the central doctrine of the New Testament. Except for limited portions that focus on the start of the Christian life by being born again, most of the New Testament is addressed to and can only be understood by Christians living "ln Christ". After that, most of what then remains of the New Testament scriptures focus on exhorting carnal Christians to become "ln Christ" Christians (see I Corinth.2-3,2 Corinth.