Etymology of "evangel"
In the sense proclaimer of the gospel, the OED derives it from the Greek εὐάγγελος (bringing good news), which in turn comes from the two elements εὖ (well) and ἀγγέλλειν (to announce).
As per Wiktionary: From Old French evangeliste, from ecclesiastical Latin evangelista, from ecclesiastical Ancient Greek εὐαγγελιστής (euangelistes, “bringer of good news”), from εὐαγγελίζεσθαι (“to evangelize”), from εὐάγγελος (euangelos, “bringing good news”), from εὖ (eu, “well”) + ἀγγέλλειν (angelein, “to announce”).
http://en.wiktionary.org/wiki/evangelist