Why One Baptism? (Ephesians 4:5 (LD 27; BC 34)

Our confession uses strong language that we are to be baptized once.  This means that we do not agree with other traditions that will baptize someone a second time because they do not see infant baptism as being a valid baptism.  Why do we believe in one baptism?  What is the bigger picture of this baptism?  How do we as Reformed people see baptism as something for children and adults?  Why are we not Roman Catholic even though we baptize infants?