Jamieson, Fausset and Brown on 1 Thessalonians 2:3

3. For--The ground of his "boldness" (1Th 2:2), his freedom from all "deceit, uncleanness, and guile"; guile, before God, deceit (Greek, "imposture"), towards men (compare 2Co 1:12; 2:17; Ep 4:14); uncleanness, in relation to one's self (impure motives of carnal self-gratification in gain, 1Th 2:5), or lust; such as actuated false teachers of the Gentiles (Php 1:16; 2Pe 2:10, 14; Jude 8; Re 2:14, 15). So Simon Magus and Cerinthus taught [Estius].

exhortation--The Greek means "consolation" as well as "exhortation." The same Gospel which exhorts comforts. Its first lesson to each is that of peace in believing amidst outward and inward sorrows. It comforts them that mourn (compare 1Th 2:11; Is 61:2, 3; 2Co 1:3, 4).

of--springing from--having its source in--deceit, &c.