God is faithful in regard to creation (Ps. 119:90), His promises (Deut. 7:9; 2 Cor. 1:18; Heb. 10:23), salvation (1 Thess. 5:24), temptation (1 Cor. 10:13), suffering (1 Pet. 4:19), and here faithful to strengthen and protect from Satan (cf. John 17:15; Eph. 6:16; 1 Thess. 3:5).
Notes for 2 Thessalonians 3:3, from the MacArthur study Bible.