Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be.
The tongue (AEsop) is at once the best and the worst of things. A man with the same breath blows hot and cold. 'Life and death are in the power of the tongue' (cf. Psalms 62:4).
Brethren - a mild appeal to their consciences by their brotherhood in Christ.
Ought not so to be - themselves may understand that such conduct deserves severe reprobation.