All 7 Uses of

apostle

in

The Scarlet Letter

The young divine, whose scholar-like renown still lived in Oxford, was considered by his more fervent admirers as little less than a heavenly ordained apostle, destined, should he live and labour for the ordinary term of life, to do as great deeds, for the now feeble New England Church, as the early Fathers had achieved for the infancy of the Christian faith.
p. 81.4

apostle = important Christian missionary

At last, while attending a sick chamber, whither the Rev. Mr. Dimmesdale had been summoned to make a prayer, she learnt that he had gone, the day before, to visit the Apostle Eliot, among his Indian converts.
p. 123.5

Be, if thy spirit summon thee to such a mission, the teacher and apostle of the red men.
p. 133.9

My one sufficient object was to greet that pious friend of mine, the Apostle Eliot, and rejoice with him over the many precious souls he hath won from heathendom!
p. 148.9

"Welcome home, reverend sir," said the physician "And how found you that godly man, the Apostle Eliot?"
p. 150.0

My journey, and the sight of the holy Apostle yonder, and the free air which I have breathed have done me good, after so long confinement in my study.
p. 150.1

The angel and apostle of the coming revelation must be a woman, indeed, but lofty, pure, and beautiful, and wise; moreover, not through dusky grief, but the ethereal medium of joy; and showing how sacred love should make us happy, by the truest test of a life successful to such an end.
p. 174.7 *

Definitions:

(1) (apostle as in: an apostle for the cause)
an enthusiastic early supporter of a cause or reform

or:

any important Christian missionary -- especially the first to a country or region
(2) (Apostle as in: chosen by Jesus)
Christianity: one of the original 12 disciples (followers) chosen by Jesus Christ to preach his gospel
(3) (meaning too rare to warrant focus)
Mormonism: In The Church of Jesus Christ of Latter-day Saints, an apostle is a priesthood office of high authority within the church hierarchy.