The most common Christian reference is to Jesus’ 12 disciples after he commissioned them to go and preach the gospel to the world. However, some churches have other usages. Some charismatic groups refer to certain powerful leaders who oversee groups of congregations as apostles. Among evangelicals, the word can be a generic term for any Christian who is commissioned by the church to accomplish a certain mission in the world.
Uppercase when referring individually or collectively to Jesus’ Twelve Apostles, as in Peter was known as Simon before he became an Apostle. Although not one of the original 12, this applies to the Apostle Paul as well.
The Church of Jesus Christ of Latter-day Saints refers to its highest-ranking members as apostles. They belong to what is called the Quorum of the Twelve Apostles. Capitalize apostle when using as an LDS title before a name. The senior, or longest-serving, apostle becomes the church president and is then referred to by that title; capitalize president before his name but lowercase otherwise.
