To me "To be in christ" means to dedicate ones life to God. It means to submit all of our beings to God to have the kind of relationship with him in which all we do we seek his will and direction. To strive to be christ like in action and in our thoughts.
What this implies to me is that I need to build a closer relationship with christ one in which my heart and spirit is always clean so God can dwell in my mortal being because if my temple is unclean then I'm none of his.