Apostolate is simply the activity one carries out in the service of the Gospel. It is being an apostle. While there may be some technical difference I’m not aware of, it is also known as “evangelization,” and I use the terms interchangeably. Around Opus Dei you are more likely to hear the word “apostolate,” but as far as I’m concerned, “evangelization” means the same thing.
One reader wrote with the following observation: “Apostolate includes evangelization, but it also includes many things which would not normally be considered evangelization—for example, setting up schools, soup kitchens, programs for the homeless, job training, etc. We are apostles (we are sent) into the world to serve the world, partly by evangelization but partly by ‘simple’ Christian service.”
I suppose if there’s a difference, that’s it. Anyway, it works for me.
Members of Opus Dei are encouraged to carry out their apostolate by continually looking for opportunities to impart the spirit of Opus Dei—that is, the universal call to holiness—to their friends, family, and colleagues through conversations, doctrine classes, study clubs, retreats, evenings of recollection, etc.