If I am born again then the Spirit of God and the Spirit of Christ dwell in me via the gift of Holy Spirit. Therefore if Christ is in me then I am in him.
Study the word "BORN" as in BORN AGAIN - anagennao from the Greek prefix ana - "again" or "up", and gennao - "to give birth". It means to be give birth to again, or to be born again, and it occurs in 1 Peter 1:3,23.
piliggenesia from palin - "again" and genesis - "origin". It means to have an origin again, a new genesis (beginning) and it occurs in Titus 3:5
apokeuo - from apo "away from" and kueo "to be pregnant". It means "to give birth to," and it occurs in James 1:18.
Men tend to think there are only two things in life that are permanent which is supposedly birth and death. But as Christians we know that death also is not permanent because we will rise again - so it seems that death isn't even permanent (except for unbelievers, i.e. second death). The only thing that is permanent is BIRTH. That is why God chose the language of birth to express the truth of what He does for us when we receive salvation. Our salvation, the new birth, is irreversible, irrevocable, enduring, lasting, does not fade, deteriorate, or lost. When God says we are born of His spirit, He means it.
One can be born of the Spirit of God or born of the devil . . . . those in darkness are not born of God, i.e. whosoever is born of God is in light because that light is in him . . . whosoever is born of the devil is in darkness . . . . can't have two different parents. Can I be in the light and sin? Apparently so because if I say I have no sin I am a liar. Now - I can walk in darkness - not having fellowship with him - If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus Christ his Son cleanseth us from all sin. . . . . These verses in 1 John are regarding our walk - our status in our relationship with the Father, the Son and the household of God - NOT as born again children of God. Ought we to walk as Christ walked - YES - but our salvation is not based on that - our salvation is based on being born of the Spirit.