Like Mark 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
So the "petty" things being discussed here most certainly can't be baptism.
The petty things include worrying about an external, third party, human activity, like baptism, instead of focusing on the actual words of Jesus instructing us how to live in love and charity. I am not against baptism, and have been baptized myself. It is meaningless, however, without a life that follows the instruction of Jesus. The sermon on the mount does not mention baptism.
http://www.gotquestions.org/baptism-Mark-16-16.html