advice : someone found a bag with things inside like wallet, candies etc. that does not belong to a Jew, found in Jerusalem. It belongs to a Christian. Do I have to return it?
Absolutely you MUST return it. According to the Meiri, any Talmudic discrimination against gentiles (such as not returning lost goods), applied only to ancient heathens. According to the "gedolim", including Rabbi Yosef Eliyahu Henkin, Rabbi Yitzhak Herzog, Rabbi Samson Raphael Hirsch, Rabbi Yehiel Yaakov Weinberg, Rabbi Hayim David Halevi, Rabbi Ahron Soloveichik, and Rabbi David Zvi Hoffmann, all have ruled that the Meiri is halacha l'maaseh [practical binding law] today.
In other words, to discriminate against a Christian today, we have no warrant whatsoever. Besides, hillul hashem [desecration of G-d's name and the Jewish religion] would be enough to warrant returning it; now that you've already posted your question online (an anti-Orthodox friend of mine alerted me to your post), hillul hashem is definitely an issue.
We also learn from the Gemara, from Rabbi Shimon ben Shetah, who returned a gem to an Arab trader, that hearing "Blessed be the G-d of the Jews", is greater than any monetary gain, and overrides any halakhah discriminatory against gentiles.
See also Rabbi Ahron Soloveichik, at http://uriltzedek.webnode.com/news/rav-ahron-soloveichik-civil-rights-and-the-dignity-of-man/, who notes that in truth, there never was a permission to cheat even an ancient evil immoral heathen. Rather, as per the Meiri, cheating him was not the crime of theft (whereas cheating gentiles today IS theft), but rather, it was a different crime, viz., "The remnant of Israel shall not do inquity or speak lies" (Zephaniah 3:13).
In other words, to retain this item, you have not the slightest warrant whatsoever.
I also remember Rabbi Joseph Telushkin, in The Book of Jewish Values, saying that someone told Rabbi Telushkin that he left Orthodoxy, because his rabbi told him that a Jew does not have to return lost goods to a gentile. Rabbi Telushkin replied that this man's rabbi lied; this law only applied to ancient immoral heathens, because you cannot have a double standard in civil law (one must realize that, at least according to the Gemara, the gentiles in that time and place were quite often involved in murder, theft, adultery, etc.). But once the gentiles are willing to deal with us equitably, we have no right whatsoever to do anything less with them.
I just saw in Rabbi Dr. Yehuda (Leo) Levi's עם ישראל ואומות העולם [The Nation of Israel and the Nations of the World],
ובס' חסידים כתב (סי' שנח): "נכרי הזריז בז' מצוות שנצטוו לבני נח, הזהר מטעותן שטעותן אסור, ותשיב לו אבידה, ואל תזלזלהו, אלא תכבדהו יותר מישראל שאינו עוסק בתורה".
[My translation: And in Sefer Hasidim, par. 358: "A gentile who is punctilious in the Seven Noahide Laws, be careful [not to benefit from] his mistakes, for [benefiting from] his mistakes is forbidden. Return his lost objects to him, and do not scorn him, but rather honor him more than a Jew who does not keep the Torah."
ועי' בתורה תמימה (דברים כ"ב ג' ס"ק כ"ב ופרק ל"ג ס"ק ג') שכתב: "כל המפרשים כתבו בכלל הדין הזה [היתר אבידת הגוי] דאיירי בעובדי אלילים הפראיים" ומצא מקור לדבר בגמרא (ב"ק ל"ח.): "ראה הקב"ה שאין האומות מקיימות ז' מצוות, עמד והתיר ממונן... אבל אלה המקיימים ז' מצוות, והם רוב האומות שבזה"ז ובכל המדינות הנאורות (!) נעלה על כל ספק שדינם שוה בכל לישראל".
[My translation: And see in Torah Temimah [R' Baruch Epstein, the son of the Aruch haShulhan], where it is written: "All the commentators have written that this law [viz. returning a lost good to a gentile] is referring to uncouth and uncivilized idolaters", and he found a source for this idea in the Gemara (Bava Kamma 38): "G-d saw that the nations of the world were not fulfilling the Seven Noahide Laws, so He made their money permitted to Israel ... but those [gentiles] who keep the Seven Noachide Laws, and they include most of the nations in our day, in all the enlightened nations, there is no doubt that their law is like that of a Jew".]