What oath does Finrod refer to when speaking with Beren?

Question

In Chapter 19 of The Silmarillion, there is the following passage:

Thus Beren came before King Finrod Felagund; and Felagund knew him, needing no ring to remind him of the kin of Bëor and of Barahir. Behind closed doors they sat, and Beren told of the death of Barahir, and of all that had befallen him in Doriath; and he wept, recalling Lúthien and their joy together. But Felagund heard his tale in wonder and disquiet; and he knew that the oath he had sworn was come upon him for his death, as long before he had foretold to Galadriel. He spoke then to Beren in heaviness of heart. 'It is plain that Thingol desires your death; but it seems that this doom goes beyond his purpose, and that the Oath of Fëanor is again at work. For the Silmarils are cursed with an oath of hatred, and he that even names them in desire moves a great power from slumber; and the sons of Fëanor would lay all the Elf-kingdoms in ruin rather than suffer any other than themselves to win or possess a Silmaril, for the Oath drives them. And now Celegorm and Curufin are dwelling in my halls; and though I, Finarfin's son, am King, they have won a strong power in the realm, and lead many of their own people. They have shown friendship to me in every need, but I fear that they will show neither love nor mercy to you, if your quest be told. Yet my own oath holds; and thus we are all ensnared.'

Now, I remember the conversation Finrod with Galadriel in question; it takes place at the end of Chapter 15:

Now King Finrod Felagund had no wife, and Galadriel asked him why this should be; but foresight came upon Felagund as she spoke, and he said: 'An oath I too shall swear, and must be free to fulfil it, and go into darkness. Nor shall anything of my realm endure that a son should inherit.'

I think I missed something in between, or misunderstood Finrod's words. The way I understood it, at the end of Chapter 15, he still did not make the oath, which he predicted he will take; during the passage I quoted in Chapter 19, he seems to have already made this oath, which now forces him to help Beren, despite the ruin he thinks it will bring. But I don't recall him taking an oath in between.

Did I miss this oath, misunderstand his words, or was it simply not mentioned?

Answer 1

The oath that Finrod is referring to was a vow he made to give "friendship and aid in every need" to Barahir and all his kin in gratitude for Barahir rescuing him in the Battle of Sudden Flame. This is detailed in chapter 18 of the Quenta Silmarillion. Note that in the quote you provide:

Felagund knew him [Beren], needing no ring to remind him of the kin of Bëor and of Barahir

and so Finrod was so honorable that Beren did not even need to show the ring. This is the oath that compels Finrod to assist Beren, despite the ruin he (correctly) sees that this will bring.