You cannot become one with God.
You can only become aware that you are already one with God.
You can only experience expressions of that Oneness
    by coming into the orbit of,
    or by intersecting with these infinite expressions.
Two can never become One.

Union with God is nothing more than the ultimate Realization
   that regardless of all appearances,
  or the testimony of the five senses,
There is only One Being, One Creator, One God.

Any sadness that I temporarily experience is the belief that I can   merge and become one with someone else
  when that is an impossibility.
There is no one to become one with.
I already am that One.

Post - Becoming aware and staying aware basically consists or constitutes knowing the True Relationship of all these expressions and continually bringing to remembrance this One Great Truth.