Who called us, all believing Christians, out of the darkness of ignorance and sin into the light of truth and holiness (1 Pet 1:15), into eternal glory in Christ. This calling itself serves as a pledge or guarantee that the one called will inherit eternal glory.
God has prepared that glory for those who love Him, in Christ Jesus; cf. 1 Pet 5:1; 1 Pet 4:13; 1 Pet 1:11.
After a brief suffering: the apostle foretells that their sufferings will be short-lived. They are brief not only in comparison with the eternal glory to which they are called, but also in comparison with the general shortness of the sorrow that has befallen them. This sorrow, then, will pass after a certain time.
May He perfect you: may He make you perfect, make you more complete, fill up and correct the deficiencies that are in you. This is the general concept of perfection, or being perfected.
It is more particularly defined by the following expressions: may He establish, strengthen, and make you unshakable. These expressions form, as it were, shades of this general concept.
May He establish means may He make you firm, so that you do not sway from side to side like a reed shaken by the wind.
May He strengthen means may He make you strong, like a solid rock, wall, or fortress.
May He make you unshakable means, properly, may He found you, lay a foundation like that for a building, so that it will not fall; cf. Mt 7:25; Heb 1:10.