英文解釈

Whatever people believes, on subjects
on which it is of the first importance to believe rightly , they ought to be able to
defend against at least the common
objections

Whatever people believesが副詞節でthey
ought to be〜が主節だと思うのですが、カンマに挟まれた句がよく分かりません。
構造と訳はどうなりますか？