It's referring to Christ.
2 Corinthians 3:17-18
17 Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. 18 And we all, who with unveiled faces contemplate the Lord’s glory, are being transformed into his image with ever-increasing glory, which comes from the Lord, who is the Spirit.
2 Corinthians 4:3-5
3 And even if our gospel is veiled, it is veiled to those who are perishing. 4 The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel that displays the glory of Christ, who is the image of God. 5 For what we preach is not ourselves, but Jesus Christ as Lord, and ourselves as your servants for Jesus’ sake.
2 Corinthians 3:14-16
14 But their minds were made dull, for to this day the same veil remains when the old covenant is read. It has not been removed, because only in Christ is it taken away. 15 Even to this day when Moses is read, a veil covers their hearts. 16 But whenever anyone turns to the Lord, the veil is taken away.