Calvin on the sin of forsaking your calling
And surely the offence of John Mark was greater than it is commonly taken for. He slid not back, indeed, from the faith of Christ, yet did he forsake his calling, and was a revolt [apostate] from the same; therefore, it was a matter which might have given evil example, if he had been straightway received again into the calling from which he was slid back. He had given himself over to serve Christ upon this condition, that he should be free no longer. It was no more lawful for him to break his promise made in this behalf, than it is for a husband to leave his wife, or for a son to forsake his father. Neither doth infirmity excuse his unfaithfulness, whereby the holiness of the calling was violated. And we must note, that he was not altogether rejected of Paul; he counted him as a brother, so he would be content with the common order; he refused to admit him unto the common [public] function of teaching, from whence he fell filthily through his own fault.
On Acts 15.37