What is the sealing of the Holy Spirit?

Is the sealing of the Holy Spirit in this dispensation, a sealing on the forehead, seen outwardly, or is it experienced inwardly?

Does not John Bunyan refer to an outward sealing in the Pilgrim's Progress?

The sign of the old covenant was circumcision, an outward mark in the flesh. It is said in the case of Abraham to have been the “seal of the righteousness of the faith, which he had yet being uncircumcised” (Romans 4:9), which was of course exceptional, as infants of eight days old have no righteousness. The Lord seals His people as His own by giving them His Holy Spirit. “In whom having believed ye were sealed with that Holy Spirit of promise” (Ephesians 1:13 R.V.; see also Ephesians 4:30; and 2 Corinthians 1:22). This is not an experience subsequent to conversion, but at conversion; nor is it an outward mark on the flesh, but a spiritual reality, which no doubt will come out in life. John Bunyan’s “Pilgrim’s Progress” is an allegory: that is, a spiritual truth conveyed under the form of a literal story. We should get into no end of difficulties, if we interpreted the figures literally.

Are all sinners, for example, to wade through a literal “Slough of Despond” on their way to the wicket gate? Besides, even Bunyan must be checked by the Word of God, and no less, “experiences” that pious persons claim to have passed through. There is no such thing taught in the Bible, as a Christian “being sealed on the forehead,” and for anyone to claim such a thing is a mistaken pretension, which he should seek grace from God to abandon.