I Tertius - Of Tertius nothing more is known than is mentioned here.
Who wrote this - It is evident that Paul employed an amanuensis to write this Epistle, and perhaps he commonly did it. Tertius, who thus wrote it, joins with the apostle in affectionate salutations to the brethren at Rome. To the Epistle, Paul signed his own name, and added a salutation in his own hand-writing. Colossians 4:18, "The salutation by the hand of me Paul;" and in 2-Thessalonians 3:17, he says that this was done in every epistle, 1-Corinthians 16:21.
In the Lord - As Christian brethren.

I Tertius, who wrote this epistle - Some eminent commentators suppose Tertius to be the same with Silas - the companion of St. Paul. If this were so, it is strange that the name which is generally given him elsewhere in Scripture should not be used in this place. I have already noticed (Preface) that some learned men have supposed that St. Paul wrote this epistle in Syriac, and that Tertius translated it into Greek; but this can never agree with the declaration here: I Tertius, who wrote, γραψας την επιστολην, this epistle; not translated or interpreted it. It appears that St. Paul dictated it to him, and he wrote it down from the apostle's mouth; and here introduces himself as joining with St. Paul in affectionate wishes for their welfare.
Salute you in the Lord - I wish you well in the name of the Lord: or, I feel for you that affectionate respect which the grace of the Lord Jesus inspires. It is not clear whether the two following verses be the words of Tertius or St. Paul.

I Tertius, who (k) wrote [this] epistle, salute you in the Lord.
(k) Wrote it as Paul uttered it.

I Tertius, who wrote this epistle,.... This name is a Latin one, and perhaps the person might be a Roman, for the names Secundus, Tertius, Quartus, Quintus, &c. were common with the Romans; unless it may be thought, as it is by some, that this man was the same with Silas, who was a constant companion of the apostle; and the Hebrew word is the same as Tertius; he also is numbered among the seventy disciples, and said to be bishop of Iconium; See Gill on Luke 10:1. Whosoever he was, it is certain he was an amanuensis of the apostle, who wrote this letter, either from the apostle's notes, or from his mouth.
Salute you in the Lord. Some connect this phrase, "in the Lord", with the other, "wrote this epistle", and make the sense to be that he wrote this epistle for the Lord's sake, for his honour and glory; which he might do, though he wrote it not by inspiration, being only scribe to the apostle; but it is better connected with the word "salute", and the sense is, that his salutation was not a mere form, nor only concerned their temporal good, but their spiritual welfare; that he wished them well in the Lord, that they might have much communion with him, and larger measures of grace from him.

I, Tertius, who wrote this--"the"
epistle--as the apostle's amanuensis, or penman.
salute you in the Lord--So usually did the apostle dictate his epistles, that he calls the attention of the Galatians to the fact that to them he wrote with his own hand (Galatians 6:11). But this Tertius would have the Romans to know that, far from being a mere scribe, his heart went out to them in Christian affection; and the apostle, by giving his salutation a place here, would show what sort of assistants he employed.

I Tertius, who wrote this epistle, salute you - Tertius, who wrote what the apostle dictated, inserted this, either by St. Paul's exhortation or ready permission. Caius - The Corinthian, 1-Corinthians 1:14. My host, and of the whole church - Who probably met for some time in his house.