76 From all that We have hitherto said, you will readily understand, Venerable Brethren, why Paul the Apostle so often writes that Christ is in us and we in Christ. In proof of which, there is this other more subtle reason. Christ is in us through His Spirit, whom He gives to us and through whom He acts within us in such a way that all the divine activity of the Holy Spirit within our souls must also be attributed to Christ. "If a man hath not the Spirit of Christ, he is none of his," says the Apostle, "but if Christ be in you..., the spirit liveth because of justification."
Source: Mystici Corporis Christi (Vatican.va)