The Holy Spirit moves through that God given intelligence.  Apostles are not just the the Twelve, but every witness to the resurrection.  When the last of these died, Pastors inherited their role of witnessing the resurrection of Christ.  As for natural law, it is the province of everyone who uses reason.  It does not depend on authority, indeed when authority is used to overrule the results of reason, it is only because it has lost the argument.  

In pre-Vatican II days, there was a large infrastructure of censorship to keep all teaching in line, with scholars required to seek Imprimater and Nihil Obstat from their local ordinaries and the Holy Office overseeing it all and the Index used to condemn anyone who resisted. Vatican II and the papal decrees implementing it destroyed most of this nonsense.  Now condemnation by CDF is a badge of honor and a way to sell more books.