Okay...you wanted harsh.
There is very little "cultist" either in the cult leader format of Beliel or the power behind the throne are of Fierna.
I consider it bad policy to include both spells per level and a feature at every level [i know 2 is blank but whatever]. No amount of skwarking will convince me that it is good design. I'd skip progression on levels that give powerful upgrades.
You've got a lot of powers that give you combat benefits. There's no manipulation going on here and this gives it a somewhat White Wolf view of sex [sex = leather + explosions], which is a shame in contrast to the actual horror of Vile feats and so forth.
Level one power is less potent than the existing feat Mark of PhleghetosFiendish Codex 2 and the feat is simple to use too. I'd simply use that in place of complication.
The acolyte power is similarly meh in its application, considering the level you'll be at. You're apparently a full blooded caster so the ability to gain a limited bonus for a brief time is somewhat pointless. Sad but true, not really a spell worth casting on yourself.
It becomes mostly needless if you use Mark of Phleghetos anyway. Maybe you could use Brand of Evil at first level and Mark of Phleghetos at 4th instead.
The fire magic boost makes little or no sense as a dual track. Fierna has fire based powers, it is true, though significantly less ability than Mephistophiles, while Beliel is entirely devoted to manipulation magics in his set of powers. I'd either split it up to the two paths or ignore it because it frankly doesn't fit the characters of either all that well.
The speaking Profanities...good a place for it as any i suppose. I would've assumed that one of the other dukes would be more deserving of it but the all seem to hold their tongues rather well. I suppose it's more to do with the Law aspect of hell holding insults in check on a direct level.
Focusing on damage with the secondary spellcasting things is...out of place. That's Mephistophile's, Bel or maybe Mammon's thing. Here, we're talking hidden spells, ones that are irrisistable when in an embrace, contagious ones that cause the cult to spread and lust for the Disciple to grow...
I have no strong opinion on the capstone. That should probably be offensive, simply because that's easier to write and harder to abuse.
For Fierna: The ability to make Suggestions that are remembered as the character's own during congress.
- Healing as a result of pain based effects
- The ability to riposte against Fear and Intimidate effects so that the opponent becomes Charmed as a result of feelings of sexual dominance.
For Belial: Deal Wisdom damage via congress as the submissive loses any will or desire to resist.
- The ability to hide his intentions perfectly, displacing suspiscion and divinations onto a willing dupe [failed save type thing]
- Repeated Intimidation leads to Charm effect on victim.
Conclusion: Overall, i think it's a well thought out and synergistic class that's too powerful and completely missed it's original design brief. No mystique, no corruption, no excess even, beyond turning your opponent a slightly darker shade of ash.