He doesn’t, and, strictly speaking, neither does the Church. Christ entrusted the job of safeguarding right doctrine and sound morals to his apostles and the bishops as successors to the apostles, not to theologians. The apostles and their successors cannot do so on their own power, but because God the Holy Spirit makes it possible. God could have chosen some other arrangement, but in his sovereign freedom he prefers this one. As for theologians, they may not be responsible for safeguarding the faith, but that doesn’t mean they are not important. It is their job to study the faith and to develop human understanding of it. The magisterium of the Church determines whether or not the developments theologians propose are in conformity to the faith handed on by the apostles and their successors.
