But the Lord is faithful (πιστος δε εστιν ο κυριος).

But faithful is the Lord (correct rendition), with a play (paronomasia) on πιστις by πιστος as in Romans 3:3 we have a word-play on απιστεω and απιστια. The Lord can be counted on, however perverse men may be.

From the evil one (απο του πονηρου). Apparently a reminiscence of the Lord's Prayer in Matthew 6:13 ρυσα ημας απο του πονηρου. But here as there it is not certain whether του πονηρου is neuter (evil) like to πονηρον in Romans 12:9 or masculine (the evil one). But we have ο πονηρος (the evil one) in 1. John 5:18 and του πονηρου is clearly masculine in Ephesians 6:16. If masculine here, as is probable, is it "the Evil One" (Ellicott) or merely the evil man like those mentioned in verse 2. Thessalonians 3:2? Perhaps Paul has in mind the representative of Satan, the man of sin, pictured in 2. Thessalonians 2:1-12, by the phrase here without trying to be too definite.