Final answer:
The third moral principle forbidding gender change in Christian moral theology is derived from the belief in natural law as divinely ordered, with gender being an unchangeable aspect of God's creation. Altering gender is seen as a deviation from this order and a challenge to God's sovereignty.
Step-by-step explanation:
The third moral principle that forbids gender change, according to some Christian moral theologians, pertains to the natural law as it is understood within the context of Christian doctrine. This principle considers gender as a given aspect of a person's nature, assigned by God, and therefore should not be altered. The concept of natural law is seen as stemming from God's order, and altering one's gender could be viewed as a deviation from this divinely ordered nature.
As referenced in various historical Christian writings, the alteration of gender is associated with deceit and a lack of modesty, which contrasts with the sobriety and sincerity expected of individuals who live by God's wisdom. Notably, Tertullian links gender expression to clothing and the outward appearance of modesty in adherence to divine will. This is rooted in the belief that God is the creator of mind and body, and changing one's gender challenges God's sovereignty and the inherent nature of human beings as created by Him.