Jamieson, Fausset and Brown on 1 Thessalonians 4:2

2. by the Lord Jesus--by His authority and direction, not by our own. He uses the strong term, "commandments," in writing to this Church not long founded, knowing that they would take it in a right spirit, and feeling it desirable that they should understand he spake with divine authority. He seldom uses the term in writing subsequently, when his authority was established, to other churches. 1Co 7:10; 11:17; and 1Ti 1:5 (1Th 4:18, where the subject accounts for the strong expression) are the exceptions. "The Lord" marks His paramount authority, requiring implicit obedience.