evangelism is the spreading of a truth or principle through earnest advocacy and personal witness, and in a second recorded sense the militant or crusading zeal with which that spreading is carried out.
The first sense is religious in origin and still primarily so: the preaching of the gospel, whether from a pulpit, a tent, or a doorstep. The second sense is what the word became when it left the church. Secular usage keeps the structure and drops the theology — a technology evangelist, a developer evangelism team, a brand evangelist — and what carries over is the specific mechanism, which is testimony. The evangelist's argument is that they were persuaded, and the persuasion is offered as the evidence. This is why the role resists being fully professionalized: an evangelist who is visibly assigned to the position loses the thing that made the testimony worth anything.
The word dates to the 1620s in the sense "the preaching of the gospel," formed from evangel plus -ism or taken from Medieval Latin evangelismus. Behind it stands evangelist, in English from the late twelfth century as a name for Matthew, Mark, Luke, and John, from Greek euangelistes, literally "bringer of good news" — eu-, "good," plus angellein, "to announce," from the same root that gives English angel. The word means messenger before it means anything else.
Evangelism is distinct from 📝Propaganda, which is centrally directed, conceals whose interest it serves, and does not require its authors to believe it. It is also distinct from ordinary advertising, which pays for its reach. The zeal in the second dictionary sense is where the word turns pejorative — the point at which advocacy stops registering an audience and becomes a campaign conducted at one. 📝Evangelism as a Service (EaaS) borrows the first sense deliberately, treating earnest personal witness as a distribution method.