Romans 16:22

I Tertius, who write the epistle, salute you in the Lord.

Tertius... means "third", many Roman names having been formed from the ordinal numbers, such as Primus, Segundus, Tertius, Quartus, Quintus, Sextus, Septimus, Octavius, etc. This Tertius was Paul's amanuensis the person who transcribed Paul's dictation, that usually having been the manner of Paul's writing. He customarily wrote a few lines at the end of his epistles with his own hand as a kind of signature. However, Galatians was written entirely by himself as he said:

Ye see how large a letter I have written unto you with my own hand (Galatians 6:11).

We are indebted to Hodge for this:

In order to authenticate his epistles, he generally wrote himself the salutation or benediction at the close; 1 Corinthians 16:21, "The salutation of me, Paul, with mine own hand"; 2 Thessalonians 3:17, "The salutation of Paul with mine own hand; which is the token in every epistle: so I write."[39]

Tertius was a Christian, and Paul honored him by asking that he write his own salutation to the brethren in Rome, which he did in these few words. Some have wondered at Tertius' greeting coming so far from the end of the letter; but such may be easily explained, either upon the probability that Paul wrote the rest of the: epistle himself with his own hand, or that there was a pause, or break, in the dictation at this point where the personal greetings were being included, before Paul proceeded to dictate the magnificent final doxology. Tertius' greeting belongs here where it was placed; and the custom of modern secretaries who type their initials at the very bottom of business letters does not reflect at all against the logic and appropriateness of the placement of Tertius' salutation.