The verb energeō (from en, in + ergon, work) means to be operative or active within, to work effectively, or to produce results. It is the root of the English word 'energy.' In the NT it is used of God, the Spirit, faith, and also of sin and the enemy — describing any power that produces effects from within.
Philippians 2:13 is one of the most powerful statements of divine sanctification: 'For it is God who works [energeō] in you to will and to act in order to fulfill his good purpose.' God is the energizing power behind both the desire (willing) and the doing (acting) of godliness — a simultaneous refutation of passive quietism and self-generated moralism. Galatians 2:8 uses energeō of God's apostolic operation: 'The one who was at work [energeō] in Peter as an apostle to the circumcised was also at work in me as an apostle to the Gentiles.' Galatians 5:6 reframes what matters before God: 'the only thing that counts is faith expressing itself [energeoumenē] through love' — faith is not inert belief but active, energized love. 1 Thessalonians 2:13 praises the Thessalonians because they received the gospel as God's word, 'which is indeed at work [energeitai] in you who believe.'