Learn how to be a policeman, because that cannot be improvised. As regards being pope, you will see later. Anybody can be pope; the proof of this is that I have become one.

The remark plays on the contrast between technical competence and spiritual office. John XXIII suggests that some roles (like policing) require concrete training and cannot be faked, while the papacy—despite its immense symbolic weight—depends less on specialized technique than on character, humility, and a willingness to serve. The self-deprecating punchline (“the proof… is that I have become one”) fits his public persona as a pastorally minded, disarming communicator: it lowers the temperature around authority and implies that holiness and leadership are not the preserve of an elite caste. Read this way, the line is both a joke and a lesson in modesty about power.