This article testifies that Marranos need to convert to be called Jewish

[edu]

http://www.aish.com/sp/so/48907402.html

Like most of the others who returned to their origins, Maria came to Judaism through her grandfather. "I was born and raised in a Catholic family in Barcelona," she says. "We were a warm, close family. We children all loved Grandpa, but if I may boast a bit, I enjoyed an extra-special relationship with him. Sometimes I would sleep over at his house when I was little, and he would hold me on his lap very early in the morning and show me the morning star shining before dawn. I can still hear his words ringing in my ears: 'Maria, do you see that star? It is the brightest star of all, and it shines before the sun rises.' Then his voice would drop, and he would sound mysterious as he said, 'Some day you'll understand why you, too, should be called by that star's name and shine as it does.'"
Maria grew, and her grandfather grew old, too old to hold his grandchildren on his lap anymore. Still, his special affection for her remained. Eight years ago, he passed away. Knowing his time was coming, he asked Maria to come and see him. "Five hundred years ago," he told her, "our family was a Jewish family, living right here. Then Ferdinand and Isabella decided to expel every Jew who wouldn't convert to Christianity. Our ancestors chose not to pack their bags and go to Europe, the Middle East, or North Africa, but rather to keep the Jewish commandments in secret. From generation to generation they have passed on the word that we are Jews, and now I want to make another link in the chain. You, too, are descended from the Jewish people, Maria, and perhaps one day you will light up the darkness like the morning star. Now I can die in peace." Those were his last words on this earth.
Without delay, Maria began investigating the meaning of Jewishness. She soon learned that Judaism was fascinating and beautiful, and she, too, is scheduled for an Orthodox conversion in the near future. She says she is sure her grandfather and all her ancestors in the upper world are overjoyed.
It is no simple matter for a young Catholic woman from Barcelona to get up one morning, cast off her whole former life, and go into the desert like Abraham, our Forefather.
Most of the countless descendants of Marranos found all over the globe have to undergo formal Orthodox conversion if they wish to be reunited with the Jewish people. Stories of their grandmothers who apparently came from Marrano families, even maternal grandmothers, cannot guarantee that over a period of 500 years or more the family never assimilated. Marrano descendants in Majorca, Belmonte, and elsewhere, have a greater likelihood of being authentic Jews, since they have historically been very particular to marry only within the extended family. They have also been rejected, over hundreds of years, by their Catholic neighbors, who called them names like chuetos that discriminated between them and the rest of the population. Some families among them have distinctively Jewish names which were kept over the centuries. Nevertheless, anyone from these localities who wants to return to the Jewish fold has to go through an Orthodox Rabbi.
The Allure of Tehillim
It might have been difficult to believe some of the stories we heard from Rabbi Ginerman and Professor HaLevi if not for the fact that they included full names, addresses, pictures, and even living voices willing to come and tell us their tales in person.
The story of a girl named Chere, for example, contains all the elements of a rather sensational novel. Chere was born in S. Klaus, Bolivia, about 20 years ago. Her father had emigrated from Germany, and her mother was a native Bolivian. Both parents were university graduates, and Chere grew up in a comfortable environment. She first heard of the existence of Jews at the age of 15,, and she took an immediate interest in them, even though she personally knew no Jews, and knew of none living within a radius of a hundred miles from her home. It was as if she'd been infected by some inexplicable fever.
She kept asking her father and mother if they were sure they had no connection to Judaism, and they repeatedly assured here they had none. Chere herself couldn't say why she felt so interested in Judaism, of all religions in the world. She found a Spanish translation of Psalms, and read them hungrily. "I felt this tremendous sweetness every time I read that book," she would say afterwards. "I was drawn to the words as if under a spell."
For three years she continued her vague, confused search after she-knew-not-what. On graduating from high school, she decided, as the daughter of a German citizen, to study medicine in her father's native land. Her parents willingly made all the arrangements for her. The night before her flight to Frankfurt, she brought up that nagging question again: "Is our family connected in any way to the Jews?"
Her parents sat facing her, perplexed, until her father broke the silence and said, "I am a Jew. My whole family perished in the Holocaust. I survived. I was a little refugee boy. When I grew up, I decided I would run away as far as I could, tell no one I was Jewish, forget I ever was Jewish, and assimilate. I wanted to escape from everything that had happened to my family."
Chere was stunned. She could hardly take in the revelation. Then she noticed that her mother had turned pale and was lying back in her chair. For her, too, this was news. They had spent over 20 years together, and he'd never told her he was a Jew. Taking a sip of water to keep from fainting, choked with emotion, she announced, "It looks like this is a night for revealing family secrets! I have a secret to tell you, too. I am probably Jewish myself! My grandmother on my mother's side told me before she died that we are descended from Jews who were forced to convert to Christianity. I never told this to anyone, not even to you," she said, turning to her husband. Then she began telling all the stories she had heard from her grandmother, who had cautioned her that this was a family secret, which must be passed on secretly to every generation to come.
So now the mystery was solved. Now Chere knew why she was so drawn to Judaism. Two powerful magnets had been pulling her in that direction all along. All that night, she sat up, crying over the fate of her ancestors and over her own unknown fate. After a sleepless night for the whole family, her parents drove her to the airport and said goodbye. She had a flight to Sao Paulo, Brazil, to connect with her plane to Frankfurt. Feeling lost and confused, she wandered around the huge airport. Suddenly she noticed a strange sight. A bearded man, dressed in black, with a large velvet yarmulke on his head, was standing in line waiting to check in. This was none other than Rabbi Daniel Ginerman, on his way to teach Torah in Frankfurt as part of the program of his Kollel in Givat Ze'ev.
"Suddenly a girl came running up to me," he says. "She was almost overcome with emotion. She burst right out and asked me, 'Are you a Jew?' I said I was, and she almost shouted for joy, 'What a story I have to tell you! I just found out a few hours ago that my parents are Jewish, and you are the first real Jew I've ever met in my life!' For the next ten hours she asked me questions about Jews and Judaism. I have never seen such eagerness, coupled with such a thirst for knowledge and wisdom. She never rested for a moment, and never seemed tired. She wanted to hear more and more. The essence of being Jewish, the mitzvot, the history of the Jewish people, their destiny, where are all the Jews today, and what did she have to do to become a proper, observant Jew according to Jewish Law. I glanced out the plane window at the cloudy sky and thought to myself that somewhere out there in Heaven, Chere's ancestors must be cheering her on."
Rabbi Ginerman had to hurry straight to his work in Frankfurt, so he put Chere in touch with one of the leading rabbis in the Jewish community there, and with another rabbi in Berlin. Today, Chere is studying Judaism in preparation for an Orthodox conversion (due to her mother's uncertain status), and she can hardly wait to return to the nation of her forbears. She talks all the time about her Heavenly-ordained meeting with Rabbi Ginerman, who was sent to the airport that day not only by his kollel, but by a guiding Hand from above.

[edu]

Pirkei Avot /Ethics of the Fathers 3:14. as translated by http://www.chabad.org/library/article_cdo/aid/682516/jewish/English-Text.htm

He would also say: Beloved is man, for he was created in the image [of G‑d]; it is a sign of even greater love that it has been made known to him that he was created in the image, as it is says, "For in the image of G‑d, He made man" (Genesis 9:6). Beloved are Israel, for they are called children of G‑d; it is a sign of even greater love that it has been made known to them that they are called children of G‑d, as it is stated: "You are children of the L-rd your G‑d" (Deuteronomy 14:1). Beloved are Israel, for they were given a precious article; it is a sign of even greater love that it has been made known to them that they were given a precious article, as it is stated: "I have given you a good purchase; My Torah, do not forsake it" (Proverbs 4:2).

The knowledge that someone is a part of the nation of Israel on some people has a powerful impact of leading them to Teshuva.
It is possible that Pirke Avot is also making this point when it says, "it is a sign of even greater love that it has been made known to them that they are called children of G‑d"
Here's another story of this nature
http://www.independent.co.uk/news/people/hungary-mep-antisemitic-moves-to-israel-after-discovering-jewish-identity-csanad-szegedi-a7325451.html

A former MEP for a Hungarian far-right party is emigrating to Israel four years after he discovered he had Jewish roots.

Csanad Szegedi had been a rising star in the anti-immigration party Jobbik, which had been accused of anti-Semitism, when he admitted his grandmother was a Holocaust survivor in 2012.

Following the discovery of his family’s tragic history, the 34-year-old renounced all ties to the party and converted to Judaism.

Now he told Hebrew language newspaper Ma’ariv he is waiting to become an Israeli citizen so he contribute to the fight against anti-Semitism.
Mr Szegedi, who had previously claimed Jewish people were “buying up the country”,  said: “Israel is an amazing country, and I believe that every Jew who lives in the Diaspora seriously considers making aliya (emigrating) to Israel, at least once in his life.

“There are many more positive elements than negative elements in being a Jew, and the biggest gift for any Jew is the existence of the State of Israel.

“After the nightmares that my relatives underwent in the Holocaust, my family and I very much want to be part of the positive dream that Israel constitutes for us.”

He refused to rule out participating in Israeli politics saying he followed it “closely” and had the “political bug” but had not joined any political party yet.
He said: “Right now, I am acting in the arena that I am familiar with, Hungary and Europe, in order to raise awareness on the issue of anti-Semitism and to work for the betterment of Israel, as a sort of compensation for the past.

“However, I have a lot of years of experience in politics and I would be happy to contribute my experience to Zionist organizations in Israel as well.”

Mr Szegedi, who was a member of Jobbik between 2006 and 2012, was reportedly also a member of the Hungarian Guard.

The group, which was banned by the courts in 2009, formed in 2007 and wore black uniforms with striped flags recalling the Arrow Cross - a pro-Nazi party which governed the country during the Second World War.
In all, an estimated 550,000 Hungarian Jews were killed during the Holocaust - most of them being sent to death camps such as Auschwitz.

Mr Szegedi, who was raised as a Christian, spoke of his shock when he was confronted about his origins in 2010.

He initially tried to cover it up but later said he had spoken to his grandmother about her experiences and realised the Holocaust had actually happened.

He said: “It was then that it dawned on me that my grandmother really is Jewish

I saw a video which claimed his maternal grandmother was jewish and therefore halachicly Jewish, having no need for conversion.
Or maybe the Rabbis asked him to convert, just in case his grandmother was lying. The facts need further investigation.

[Zelhar]

doesn't "Marranos" mean pigs in spanish?