I will answer. First of all "baptism" the word or related words is NOT most often used in reference to the Spirit. It's used most often of water baptism. When the word is used, it's often used in the full description so as to distinguish it...ie it's worded "baptize you with the Holy Ghost and with fire"...so when baptism of the Spirit is being spoken of the word baptize and Spirit are used in conjunction...
In the given text, however, it would be redundant to say "one baptism" and then say "one Spirit" as separate articles....why not rather "one Spirit baptism" or "One baptism of the Spirit"? Why say "One Lord, one faith and One baptism"
Eph 4:4 There is
one body, and one Spirit, even as ye are called in one hope of your calling;
Eph 4:5 One Lord, one faith, one baptism,
In fact, when Paul mentions body and Spirit in 1Corthians he is speaking specifically of Spirit baptism
1Co 12:13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.
So why say "one Spirit" and then "One baptism" if referring to the same thing?
Seems redundant.
One commentary says under the law there were many baptisms....
and also we see from Acts there was Johns baptism, but Paul re-baptized those men according to Christian baptism...if Johns baptism was acceptable that would make for more than one baptism.
Seems clear to me he is referring to water baptism and the various modes floating around. John's, the law, the Essenes....or Christian baptism for us...just one