The focus today is on verse 13, as it gives purpose to the latter part of verse 12,"...building up the body of Christ.."
We are given gifts, through the Holy Spirit, according to the measure and the will of Christ. We use these gifts to engage the world for Christ and thereby give increase, as well as to strengthen one another. This is the short explanation of verse 12. But what does it mean to "...build up the Body of Christ..."? The question is answered in verse 13 of Ephesians 4:
"until we all attain to the unity of the faith and of the knowledge of the Son of God - a mature person, attaining to the measure of Christ's full stature. " The NET Bible (http://net.bible.org/#!bible/Ephesians+4:13)
μεχρι καταντησωμεν οι παντες εις την ενοτητα της πιστεως και της επιγνωσεως του υιου του θεου εις ανδρα τελειον εις μετρον ηλικιας του πληρωματος του χριστου WHNU
"until we all might attain into a unity of faith and of the knowledge of the Son of God, into a complete man, in a measure of maturity of the fullness of Christ" MP (my translation of the Greek)
There are three prepositional phrases in this verse, and each one begins with the "dynamic" "in", the Greek proposition εις. Why do I say "dynamic"? This form of "in" requires the accusative form of the noun, implying motion. The "at rest" form of "in" for Greek is εν, which requires the dative form of the noun. An analogy of the English language could be "He is sitting in the room", for which the Greeks would use εν and the dative, as opposed to "He is going in the room", for which the Greeks would use εις with the accusative.
Each prepositional phrase in verse 13 has the believers moving toward a goal, a goal which also, in the abstract, will require their immersion in that goal ("in" still means "into" something).
Goal one: a unity of faith and of the knowledge of the Son of God
Our spiritual gifts, our calling into specific offices of the Body, these should bring the outcome of unity in faith, a harmony of belief and believers. The exercise of these gifts and offices should continue to increase our intimate knowledge of Christ.
Goal two: a complete man
As we learn, exercise our faith, and strive in our faith, through the transforming power of the Holy Spirit, we should become "a complete man". That is, our weaknesses are brought under control, our strengths are checked, we develop balance and skill in all areas of ministry, and we grow in our intimacy with Christ.
Goal three: a measure of maturity of the fullness of Christ
It is expected that part of what we attain is the likeness of Christ, through Him working within us. The idea here is that we are maturing, progressing, becoming more immersed in the fullness of Christ, and thereby reflect an increasing portion of Him in us.
Keep reaching, keep striving, keep attaining... and unto Him be all the glory.