Jamieson, Fausset and Brown on Ephesians 6:18

18. always--Greek, "in every season"; implying opportunity and exigency (Col 4:2). Paul uses the very words of Jesus in Lu 21:36 (a Gospel which he quotes elsewhere, in undesigned consonance with the fact of Luke being his associate in travel, 1Co 11:23, &c.; 1Ti 5:18). Compare Lu 18:1; Ro 12:12; 1Th 5:17.

with all--that is, every kind of.

prayer--a sacred term for prayer in general.

supplication--a common term for a special kind of prayer [Harless], an imploring request. "Prayer" for obtaining blessings, "supplication" for averting evils which we fear [Grotius].

in the Spirit--to be joined with "praying." It is he in us, as the Spirit of adoption, who prays, and enables us to pray (Ro 8:15, 26; Ga 4:6; Jude 20).

watching--not sleeping (Ep 5:14; Psa 88:13; Mt 26:41). So in the temple a perpetual watch was maintained (compare Anna, Lu 2:37).

thereunto--"watching unto" (with a view to) prayer and supplication.

with--Greek, "in." Persevering constancy ("perseverance") and (that is, exhibited in) supplication are to be the element in which our watchfulness is to be exercised.

for all saints--as none is so perfect as not to need the intercessions of his fellow Christians.