What the Bible says about Baptismal Formula
(From Forerunner Commentary)
Trinitarians suggest that the baptismal formula in Matthew 28:19 sets a precedent for believing in the Trinity, since, within it, Father, Son, and Holy Spirit are juxtaposed. If we consider this verse more closely, we see the prepositions "in" (or "into") and "of." Taken together in this idiom, they suggest that the last item in the series would be the means by which the act can take place. Thus, people are baptized into the name of the Father and Son by means of the Holy Spirit.
David F. Maas
Misconceptions and Malarkey About the Holy Spirit (Part One)