The noun thrēskeia denotes religious practice — the external forms, rites, and observances that express one's devotion to a deity. It refers to religion as practiced and performed, not as believed internally. In NT usage it appears in both positive and negative contexts, depending on whether the practice is genuine or self-serving.
James 1:26–27 contains the NT's most incisive definition of true thrēskeia: 'If anyone considers himself religious [thrēskos] and yet does not keep a tight rein on his tongue, he deceives himself, and his religion [thrēskeia] is worthless. Religion [thrēskeia] that God our Father accepts as pure and faultless is this: to look after orphans and widows in their distress and to keep oneself from being polluted by the world.' James demolishes the separation between vertical piety and horizontal ethics: true thrēskeia is measurable in care for the vulnerable. Acts 26:5 uses thrēskeia to describe Paul's pre-conversion life as a Pharisee — external religious rigor without the Spirit's transforming work. Colossians 2:18 warns against those who delight in the 'worship [thrēskeia] of angels' — a self-made religion that may appear spiritual but lacks connection to Christ as the Head.