Canon Law
Which is also called written reason is the governing law not only of the Catholic Church, but of the Vatican. As such to be a citizen of the Vatican and even moreso its head, you must be a members of the Catholic Church as prescribed by Canon Law.
Amleto Cicognani in Canon Law (1935) teaches: However, on account of the divine assistance which Christ promised His Church, no disciplinary law at variance with orthodox faith or good morals has ever been or ever will be issued by the Roman Pontiff for the universal Church.
The laws issued by the Holy See are promulgated by being published in the official organ of the Holy See, the Acta Apostolicae Sedis, unless in particular cases another mode of promulgation is prescribed. (Canon 9)
Henry Cardinal Manning, The Internal Mission of the Holy Ghost, pages 191-2: The sacred canon law against which the rebellious wills and shallow intellects of men have ever clamoured is the noblest, highest, purest legislation that mankind has ever known. The jurisprudence of the Church is the perfection of wisdom and justice. And here the difference between the Church and the world comes out into light. The doctors and legislators of the world may be unsanctified men. The doctors and law-givers of the Church are created by the Holy Ghost. Of the men of science who are the doctors of the world at this day, many openly deny the existence of God. We are living in a century when men pride themselves on their intellectual culture, and on the illumination of science, and I know not what; but it is an intellectual culture which prides itself upon its independence of God, and its divorce from the sanctification of the will. The whole power of ‘progress and modern ‘civilisation’ is at this moment attempting to accomplish four things-to expunge God from science, to drive His name our of education from the highest to the lowest school, to shut out His revealed law from the whole public order of the states, and from the whole culture of the human intellect. What wonder it the apostles and prophets, the doctors and Pharisees, of modern civilization rose up and threw dust in the air at he promulgation of the Syllabus? But I may not go on. The Holy Ghost creates saints, and the Holy Ghost creates doctors, but fools create themselves.
Heretics are not members of the Catholic Church.

Michael Fighting
"Michael…who standeth for…thy people," — Dan. 12:1-12