Apostle Paul said in Galatians 4:19, “My children, with whom I travail again in birth until Christ is formed in you,”
So how is Christ formed in us. I will explain this way; And the angel answered and said to Mary,”The Holy Spirit will come upon you, and the power of the Highest will overshadow you;(Luke 1:35)
Christ being formed in us is Christ being revealed in us. Just as it was for Mary, Christ is revealed in us when we submit our-self completely to the power of the highest (the Holy Spirit). When we let go and let God. At that point, Christ can be expressed in us to His glory.