Jamieson, Fausset and Brown on 1 Peter 3:4

4. But--"Rather." The "outward adornment" of jewelry, &c., is forbidden, in so far as woman loves such things, not in so far as she uses them from a sense of propriety, and does not abuse them. Singularity mostly comes from pride and throws needless hindrances to religion in the way of others. Under costly attire there may be a humble mind. "Great is he who uses his earthenware as if it were plate; not less great is he who uses his silver as if it were earthenware" [Seneca in Alford].

hidden--inner man, which the Christian instinctively hides from public view.

of the heart--consisting in the heart regenerated and adorned by the Spirit. This "inner man of the heart" is the subject of the verb "be," 1Pe 3:3, Greek: "Of whom let the inner man be," namely, the distinction or adornment.

in that--consisting or standing in that as its element.

not corruptible--not transitory, nor tainted with corruption, as all earthly adornments.

meek and quiet--meek, not creating disturbances: quiet, bearing with tranquillity the disturbances caused by others. Meek in affections and feelings; quiet in words, countenance, and actions [Bengel].

in the sight of God--who looks to inward, not merely outward things.

of great price--The results of redemption should correspond to its costly price (1Pe 1:19).