John Calvin Commentary 1 Thessalonians 4:9

"But concerning love of the brethren ye have no need that one write unto you: for ye yourselves are taught of God to love one another;" — 1 Thessalonians 4:9 (ASV)

As to brotherly love. Having previously, in lofty terms, commended their love, he now speaks by way of anticipation, saying, you do not need me to write to you. He assigns a reason—because they had been divinely taught—by which he means that love was engraved upon their hearts, so that there was no need of letters written on paper.

For he does not mean simply what John says in his first epistle, the anointing will teach you (1 John 2:27), but that their hearts were framed for love; so it appears that the Holy Spirit inwardly dictates efficaciously what is to be done, so that there is no need to give injunctions in writing.

He adds an argument from the greater to the less; for as their love spreads through the whole of Macedonia, he infers that it is not to be doubted that they love one another. Hence the particle for means likewise, or even more, because, as I have already stated, he adds it for the sake of greater intensity.