The fullness of the Spirit is mystical, but not indefinite.  The Holy Spirit is simultaneously the Spirit of God and the Spirit of Christ.  Thus He comes to indwell us with the fullness of God.  Jesus is the image of the invisible God and is the image meant to be the guiding star for how we live.  The fruit of the Spirit’s fullness is the character of Christlikeness and is the real evidence that God lives in us.

“That He would grant you, according to the riches of His glory, to be strengthened with power through His Spirit in the inner man, so that Christ may dwell in your hearts through faith; and that you, being rooted and grounded in love, may be able to comprehend with all the saints what is the breadth and length and height and depth, and to know the love of Christ which surpasses knowledge, that you may be filled up to all the fullness of God.” Ephesians 3:16-19 (NASB)