Our faith is tested by the fire for a purpose—that it may be found to praise, honor, and glory at the revelation of Jesus Christ, whom having not seen you love. Though now you do not see Him, yet believing, you rejoice with joy inexpressible and full of glory (1 Pet. 1: 8). Faith is refined so that at the last day, at the final consummation of the kingdom of Christ, it will be the occasion for praise, honor, and glory. God values your faith more than He values your gold or your present comfort. Peter is moved by the fact that the readers of his epistle love Christ, despite never having seen Him. Our Lord Himself said, “Blessed are those who have not seen and yet have believed” ( John 20:29). After the resurrection when Jesus appeared to the eleven in the upper room, He rebuked them for their unbelief, for their hardheartedness. They had not believed the testimony of the angel and the women who were at the tomb. God places a premium on faith that is the substance of things not seen, as the author of Hebrews indicates (Heb. 11:1).
Inexpressible joy is a reality that human words can never adequately describe. That joy, which is a fruit of the Holy Spirit, is ineffable. It defies description. One commentator on this text likened it to the glory of the Son. He said, “A blind man who has been blind from birth cannot understand the noonday sun. No matter how many times you try to explain it to him, he has no reference point by which to understand its magnitude.” The author went on to say that someone who can see may not be able to express adequately the reality of the brightness of the sun to someone who is blind, but the person who can see knows the sun the moment it shines upon him. We perceive the light. We do not have to reason about it; we see it for what it is. So it is with the Word of God. Many people are blind to the truth of God, but when the scales fall from their eyes and the Spirit of God opens their eyes to His Word, they see the truth of it immediately. We certainly have sound, objective reasons to believe the Word of God, but those reasons are about as necessary as arguments for light to people who can see the sun. Our joy is inexpressible. It is a glorious joy, a weighty joy, not a superficial joy.
R.C. Sproul, 1 & 2 Peter: St. Andrew’s Expositional Commentary, p. 38 (Kindle Edition)