Follow peace with all (i.e. as required by the context, with all the brethren; cf. Romans 14:19), and holiness (more properly, sanctification), without which no man shall see the Lord. Here the figure is dropped, and two cautions given, peculiarly needed, we may suppose, by the community addressed.
The exhortation to "peace with all" reminds of the tone of St. Paul's admonitions both in Romans and in 1 Corinthians, where he so strongly warns against dissensions and party spirit, and enjoins tolerance and mutual allowance with regard to the weaker brethren.
The word ἁγιασμὸς ("sanctification") need not be limited (as by Chrysostom) to the idea of chastity; the general thought implied may be (as expressed by Limborch, quoted by Alford), "No, dum pact studeat, nimis slits obsequendi studio quidquam contra sanctimonism Christianam delinquat;" but the special allusion to πορνεία in verse 16 (as also in Hebrews 13:4) is evidence that chastity was especially in the writer's mind, with definite reference to which the word ἁγιασμὸς is used in 1 Thessalonians 4:3.
The frequent and earnest warnings against fornication in St. Paul's Epistles are enough to show how slow even some in the Church were to recognize the strict code of Christian morality, unknown to the heathen world, and by the Jews very imperfectly recognized, in this regard; and the case of 1 Corinthians 5:1-13.
illustrates how easily such vice might creep into and infect a Christian community without general reprobation. Hence probably the special warning here.