"God has not called." We have been called to the confession of the Gospel, to faith in Christ, to Christianity. By saying "us," he places himself in the same rank as the Thessalonians and all Christians.
After threats, he reminds them of the spirit of Christianity. Such is the calling of Christians, according to the pre-eternal appointment: to be pure.
"For" indicates the reason why God is the avenger, especially for acts of fornication. It is because they are directly opposed to God's calling and do not so much disturb proper relations with brethren and oneself as offend God Himself.
"God will punish you," says St. Chrysostom, "not in order to avenge the brother, but because you have offended God Himself. He called you, and you have offended the One Who called."
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For this reason Christianity begins with the vow of purity and the renunciation of lustful deeds. Pagans, when receiving baptism, obligated themselves to leave these things behind and to love chastity.
Under this condition God also gives them the grace of the Holy Spirit, which requires of the recipient that the vessel of his nature be pure.
"The devil," writes blessed Theodoret, "filled the doctrines of nonexistent gods with indecency, urging men to enter this path, that is, the path of indecency. But the true God commands us to turn away from this path and prescribes that we walk the path of chastity."
Christians were sharply distinguished from pagans in this regard. Thus the latter marveled that they did not run with them into the same flood of dissipation (1 Pet 4:4).