John Calvin Commentary 2 Thessalonians 3:17

"The salutation of me Paul with mine own hand, which is the token in every epistle: so I write." — 2 Thessalonians 3:17 (ASV)

The salutation, with my own hand. Here he again guards against the danger he had previously mentioned—lest epistles falsely ascribed to him find their way into the Churches.

For this was an old artifice of Satan—to put forward spurious writings, that he might undermine the credibility of those that are genuine; and furthermore, under the pretended names of the Apostles, to spread wicked errors with the aim of corrupting sound doctrine.

By a singular kindness from God, it has come about that—his frauds being defeated—the doctrine of Christ has come down to us sound and entire through the ministry of Paul and others. The concluding prayer explains how God helps His believing people—by the presence of Christ’s grace.