The Gospel of Philip names five rites: baptism, chrism or anointing, eucharist, redemption and the bridal chamber. This is the earliest Christian text to set out a sequence of that shape, and it is the reason the book matters to historians of liturgy quite apart from anything it says about doctrine. The ranking is explicit, with the bridal chamber placed above the rest, and the anointing given more weight than the baptism that precedes it, which is the reverse of the emphasis found in most later Christian practice.
What the bridal chamber actually was is genuinely unresolved, and the disagreement is old. One line of reading takes it as a distinct ritual, performed on initiates, perhaps involving a chamber, a kiss or an anointing, and standing as the culminating sacrament of a Valentinian community. Another takes it as an image: the reunion of the divided self with its heavenly counterpart, of which any earthly ceremony is only a picture. A third position, which has gained ground, is that the distinction is anachronistic, and that a rite and the metaphor it enacts were not separable in the minds of the people who used the term.
The uncertainty is not evasion by commentators. The text nowhere describes a procedure. It speaks of the bridal chamber constantly and defines it once, and the surrounding excerpts are exactly the kind of allusive material a community writes for people who already know what is being referred to. The one firm point is that the language draws on ordinary marriage while insisting the mystery is not that of ordinary marriage, and that whatever was meant, it was held to be undefiled rather than transgressive.