In my opinion, "with spirit" is preferable on grounds that the instrumental
sense of the dative case is quite commonly used in Koine with the
preposition EN, although more commonly and earlier EN is used with a
locative dative to indicate stationary position in or on a location.
There are, however, some who argue that in the baptismal phrasings EN
hUDATI and en PNEUMATI (hAGIWi) we actually have a locative dative
indicating dunking of the convert in water and/or spirit. While that
possibility can't be ruled out absolutely, it would make more sense when
used with the verb BAPTIZW than with PLHROW, and I'm not altogether
convinced that assumptions about the proper mode of baptism aren't a factor
in the argument for the locative interpretation. In the present instance,
however, I think the locative interpretation is improbable.
Carl W. Conrad
Department of Classics, Washington University
One Brookings Drive, St. Louis, MO, USA 63130
firstname.lastname@example.org OR email@example.com