It is to Paul chiefly that we owe the thought (which is also found in John's first letter) that Christ Himself lives in men's hearts. No one could read with an open mind the Letters of the New Testament without seeing that people are being, sometimes suddenly and sometimes step by step, transformed. The reason for this, according to Paul, is an open secret. In the past, he says in effect, men have striven to please an external God; now God's great secret is plain. With the coming of the Good News, indeed it is part of the Good News, God is prepared to live within the personalities of those who use their faculty of faith towards Him. In Paul's writings we do not read of Jesus Christ as an Example who lived and died some years before and Who must be followed and imitated. On the contrary, Paul's letters are ablaze with the idea that, if men will believe it, Christ is alive and powerful, ready to enter and transform the lives of even the most unlikely. This happens, he says, "by faith". But how rarely in present-day Christianity do we meet such a faith! Many Christians do not appear to have grasped this, one of the essentials of the Gospel. It is true that they believe in God, they pray to God, and they try to follow the example of Christ. But, as far as one can tell, they have not begun to realise that Christ could be living and active at the very centre of their own personalities. And, of course, so long as they do not believe it, it is not true for them. For just as in the days of Christ's human life the divine power was inhibited or limited by the absence of faith, so His activity within the personality is limited where a man does not in his heart of hearts believe in it. If we modern Christians are steadfastly refusing to believe in this inward miracle, it is not surprising that our Christian life becomes a dreary drudge.
[J B Phillips - New Testament Christianity]


Within the 'body of Christ' nothing intervenes between the Head and the limb. Each and all, feet, fingers, all, are things that 'hold the Head', each having its own point of direct contact with the Head. The image of the body makes it clear and vivid that I live by the Lord Christ, and for Him, and in deep connexion with all His people. I live by Him directly, spirit with spirit, by the mutual contact of my faith and Him.
So one all-important aspect of Christian growth is growth inward; not in the sense of concentrating on oneself, but as growth out of self into the Lord, the ever more deeply, vividly and continuously realised centre of the plan of life, and source of 'all-sufficiency in all things'. We may be permitted to grow outward, to extensions of activity or influence or success in the work of the Lord. But whether we are led to this, or to be laid aside, narrowed and shrivelled into what seems nothing, let us see to it that we are 'growing into Him'.
[Condensed from H C G Moule Thoughts on Union with Christ]


I ask the Father in his great glory to give you the power to be strong inwardly through his Spirit. I pray that Christ will live in your hearts by faith and that your lives will be strong in love and be built on love. And I pray that you and all God's holy people will have the power to understand the greatness of Christ's love — how wide and how long and how high and how deep that love is. Christ's love is greater than anyone can ever know, but I pray that you will be able to know that love. Then you can be filled with the fullness of God. With God's power working in us, God can do much, much more than anything we can ask or imagine. To him be glory in the church and in Christ Jesus for all time, for ever and ever. Amen.
[Ephesians 3.16-21 New Century Version]