In today's Gospel reading Jesus offers us a glimpse of what his union with the Father in the Spirit is, speaks of being in the Father and the Father being in him, and the evangelist then says that Jesus "eluded their grasp."  This last expression seems to echo "the darkness could not comprehend him" of John's first chapter and foreshadows Jesus telling Mary Magdalene not to cling to him (John 20).  The first two are probably similar in meaning, indicating that Jesus simply cannot be understood, grasped, controlled, or overcome by those who wish him ill or use him.  

But if we wish to cling to Jesus in trust and love, as Mary did?  Jesus does not brush us off or push us away, merely warns us that since we are still imperfect (literally "unfinished") we cannot truly get ahold of who he is --- and might harm ourselves if we thought that we had.

That is what our trust and love is all about, knowing that the life of the Trinity is something that we will spend eternity discovering and that until then we must let the Spirit guide us and hold us in the center of the triune love.  Then we will be in the Father and the Son and the Spirit, like a child cherished in its parents' arms, and they shall be alive and active in us.  And although they will always elude our grasp, we shall not elude theirs.