Guy Kawasaki has an interesting view on what it means to evangelize a product. His official title is the chief evangelist of Canva, an online graphic design service, and he has previously worked as the chief evangelist of Apple. In his blog Kawasaki explains how evangelism is different from sales or marketing.

The key point of product evangelism is that the product must be something that you are invested in and that can change the world. You do not evangelize something that you are indifferent about, but rather something that you truly care about. The products that can be evangelized are ones that create shared value by improving quality of life for customers and society. Evangelists keep the interests of others in mind and at heart rather than just their own interests.

Evangelizing a product or a company is extremely useful, because it leads to an emotional attachment of new customers to that company or product. These customers will undoubtably continue to purchase from the company and will evangelize the company to others. Guy Kawasaki obviously did a great job with Apple, seeing as everyone knows at least one person who cares passionately about Apple products and tries to convince others that they are superior. Successful evangelism will lead to additional marketing.

Source: http://blog.canva.com/art-evangelism/

Source: http://blog.guykawasaki.com/2014/04/the-art-of-evangelism.html