zēlos

At its root this word means heat: a boiling, burning intensity of feeling. The picture is of something brought to the boil, and the same heat can warm or scald depending on where it is aimed. In this first range of meaning the heat is admirable. It is the fervor that fixes a person wholly on God, on a sacred cause, or on the good of others.

The New Testament writers inherited two streams of thought here. From the wider Greek world came the idea of a noble striving to match and honor what is excellent. From the Hebrew Scriptures came the burning loyalty of God for his people and of his people for him: the devotion of figures like Phinehas and Elijah, and God's own protective love that brooks no rival. Both streams feed these passages.

The senses below run from human zeal directed at God and the sacred, through earnest devotion shown toward people and good causes, to God's own jealous love for those who are his, and finally to that same divine ardor turned to consuming fire against his enemies. What holds them together is the heat and its worthy object: in each case the fire burns for something genuinely deserving of it.

The danger this word carries is that the very same fervor, redirected toward one's own pride or party, curdles into the jealousy described under the second meaning. Sincerity of feeling is no guarantee of rightness, as the people who had a zeal for God but lacked knowledge make plain.

Here the same boiling intensity has gone bad. Instead of burning for a worthy object, it burns to defend one's own standing, party, or pride against a perceived rival. The result is jealousy, envious rivalry, and the bitter competitiveness that tears a community apart.

In the New Testament this negative use travels in fixed company. It is paired again and again with strife, and it stands in lists of the behaviors that wreck relationships: outbursts of jealousy, factions, quarreling. The letter of James gives it the sharpest treatment, calling it a bitter jealousy that is earthly and even demonic, and tracing disorder and every vile practice back to it. In the book of Acts it is the motive of those who oppose God's messengers, filling the high priest and hostile crowds with a resentment that drives them to violence.

The two meanings are not two unrelated words but one heat pointed in two directions. The line between them is the object: a fervor aimed at God or the good of others is devotion, while a fervor aimed at protecting one's own status against a rival is jealousy. Scholars debate whether this is best described as two distinct meanings or as a single word whose value the context supplies, but in plain reading the difference between admirable zeal and destructive jealousy is real and large.