First, we must understand that as long as Christ remains outside of us, and we are separated from him, all that he has suffered and done for the salvation of the human race remains useless and of no value for us. Therefore, to share with us what he had received from the Father, he had to become ours and to dwell within us. For this reason, he is called ‘our Head’ [Eph. 4:15], and ‘the first-born among many brethren’ [Rom. 8:29]. We also, in turn, after said to be ‘engrafted into him’ [Rom 11:17], and to ‘put on Christ’ [Gal. 3:27]; for, as I have said, all that he possess is nothing to us until we grow into one body with him.
Calvin, Institutes, 3.1.1.