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Why did Kant consider God, freedom, and immortality necessary postulates of moral life?

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Immanuel Kant German philosopher Immanuel Kant surrounded by Ouroboros, an emblematic serpent of ancient Egypt and Greece, engraving by John Chapman, published in London, 1812.

In his Critique of Practical Reason, Immanuel Kant argued that while God, freedom, and immortality cannot be proven through theoretical reason or sense experience, they are necessary postulates for the moral life.1

Freedom: Kant viewed freedom, or the autonomy of the will, as the foundation of morality. To act morally, one must be able to act according to a categorical imperative that is independent of the causal determinism of the phenomenal world.2
Immortality: Kant defined holiness as the perfect accordance of the will with the moral law. Because humans cannot achieve this perfection in a single lifetime, he argued that it requires an endless progression, which is only possible under the supposition of the immortality of the soul.3
God: Kant posited the existence of God as the necessary guarantor of the moral order, ensuring that the pursuit of moral duty and the attainment of the highest good are ultimately coherent.13

These postulates are not objects of knowledge but are essential requirements for the consistency and meaningfulness of moral philosophy.1