Jamieson, Fausset and Brown on 1 Peter 5:9

9. (Lu 4:13; Ep 6:11-17; Jas 4:7.)

steadfast--Compare established in the truth," 2Pe 1:12. Satan's power exists only in respect to the unbelieving; the faithful he cannot hurt (1Jo 5:18). Faith gives strength to prayer, the great instrument against the foe (Jas 1:6, &c.).

knowing, &c.--"encouragement not to faint in afflictions": your brethren suffer the same; nothing beyond the common lot of Christians befalls you (1Co 10:13). It is a sign of God's favor rather than displeasure, that Satan is allowed to harass you, as he did Job. Your fellow Christians have the same battle of faith and prayer against Satan.

are--are being accomplished according to the appointment of God.

in the world--lying in the wicked one, and therefore necessarily the scene of "tribulation" (Joh 16:33).