Union with God, considered as a form of Christian experience, is not a physical union, an union of essence with essence physically, but a moral and religious union. It would hardly be necessary to make this remark, were it not, that some pious writers on this subject make use of strong expressions, which may be easily misunderstood and misapplied, but which obviously were not designed to be and ought not to be taken in their physical or literal import. The passages of Scripture, which recognize and which require the union of the regenerated mind of man with the mind of his Maker, or with the mind of Christ, are in some instances exceedingly strong, and seem to require a modified interpretation. All that is necessary is, that we should exhibit in other cases the discrimination and candor, which generally characterize our interpretations of the Scriptures. But, although we are not to understand from the language of the writers on this subject, that there is a physical union, or a union which would imply in any sense the loss of our own personality and accountability, they undoubtedly mean to teach the existence and the reality of a moral and religious union, as close and intimate as such an union possibly can be; an union entirely analogous, in all probability, to that pure and blessed union, which existed between Christ Jesus considered in his human nature, and his heavenly Father.
— edited from The Interior or Hidden Life (2nd edition, 1844) Part 3, Chapter 13 by Thomas Cogswell Upham. His blog is managed by Craig L Adams and can be found here: http://thomascupham.blogspot.com