(1) The enthusiastic preaching or proclamation of the Christian gospel. (2) Militant of crusading zeal.
Example:
Their evangelism for the new program won many converts among those who had previously doubted its merits.
Evangelism comes from euangelion, the Greek word for "gospel" or "good news." The firm belief that they are bringing "good news" has traditionally filled Christian evangelists with fiery zeal. Evangelism can now refer to crusading zeal in behalf of any cause. The evangelical enthusiasm of some environmentalists has won over some segments of the general public while alienating others.