when is haki introduced in one piece

Haki is introduced quite late in the series of One Piece, more specifically during the 'Sabaody Archipelago' story arc, in the conversations between Silvers Rayleigh and the Straw Hat Pirates, which is around episode 394.

The concept of Haki in One Piece can be viewed somewhat as a spiritual power – a force that emerges from the user's spirit or will. It enforces the idea of inner strength and determination, aspects that can be related to several religious and spiritual beliefs. Haki allows users to manifest their willpower into physical force.

Absolutely, the concept of Haki can be connected to the Christian virtue of faith. In essence, faith is belief and trust in God, which gives believers the strength and determination to overcome challenges in life. Similarly, Haki users rely on their inner strength and belief in themselves to overcome their adversaries.