A verb meaning to boil, be hot, or be fervent. It describes the bubbling heat of boiling liquid, used metaphorically for intense spiritual fervor or enthusiasm. Paul uses it to describe the zeal of the Spirit-filled Christian, while Revelation uses the contrast between hot and cold as a warning against spiritual lukewarmness.
The temperature of faith matters profoundly in Scripture. Being lukewarm — neither hot nor cold — is described in Revelation as more repugnant to God than being cold (Revelation 3:16). Zeo is the verb Paul uses in Romans 12:11: 'fervent in spirit, serving the Lord' — literally, boiling in spirit. This is not manufactured enthusiasm but the natural heat of a life on fire with the Spirit. The Spirit himself is described as fire — at Pentecost the tongues of fire distribute to each believer. The Christian life calls for maintained heat, guarded against the cooling effects of comfort and compromise.