Sharing our faith builds our understanding
Paul may be saying that the act of sharing our faith is first of all necessary to show we have a full understanding (because if we aren't sharing, we don't fully understand), and/or that the act of sharing our faith increases our faith and understanding of our blessings in Christ. Certainly the act of exercising our faith always increases our faith. As we are obedient and step out, God shows himself faithful, thereby increasing our knowledge of him and our faith.
