The findings of this research suggest a compatibility between Fenelon’s Spiritual Letters and Existential theories of personal growth. The Psychotherapy and Counselling Codes of Ethics in Australia, America, and England emphasize the importance of personal growth as an essential tool to ensure responsibility and quality in professional practice. Self-knowledge and self-development are consistent with the ethical principles of Fidelity, Autonomy, Beneficence, Non-maleficence, Justice, and Self-respect. Enriching self to deliver better quality and high ethical service in mental health should be seriously considered for both the trainee and the experienced psychotherapist. The similarities between Fenelon Spiritual Letters and Existential Psychotherapy may suggest them as a possible tool for inner development, subject of course to further empirical research.