Jamieson, Fausset and Brown on 1 Thessalonians 5:12

12. beseech--"Exhort" is the expression in 1Th 5:14; here, "we beseech you," as if it were a personal favor (Paul making the cause of the Thessalonian presbyters, as it were, his own).

know--to have a regard and respect for. Recognize their office, and treat them accordingly (compare 1Co 16:18) with reverence and with liberality in supplying their needs (1Ti 5:17). The Thessalonian Church having been newly planted, the ministers were necessarily novices (1Ti 3:6), which may have been in part the cause of the people's treating them with less respect. Paul's practice seems to have been to ordain elders in every Church soon after its establishment (Ac 14:23).

them which labour ... are over ... admonish you--not three classes of ministers, but one, as there is but one article common to the three in the Greek. "Labor" expresses their laborious life; "are over you," their pre-eminence as presidents or superintendents ("bishops," that is, overseers, Php 1:1, "them that have rule over you," literally, leaders, He 13:17; "pastors," literally, shepherds, Ep 4:11); "admonish you," one of their leading functions; the Greek is "put in mind," implying not arbitrary authority, but gentle, though faithful, admonition (2Ti 2:14, 24, 25; 1Pe 5:3).

in the Lord--Their presidency over you is in divine things; not in worldly affairs, but in things appertaining to the Lord.