That is false. The Serbs and the Croats are not the same tribe. They are all Slavs, of course, as much as Norwegians and Austrians are all "German."
The Serbs and the Croats were not divided by religion -- in fact religion (Roman Catholicism) was not an identifier of nationality until Maria Theresa's times. It was the language.
But, you will observe, don't Serbs and Croats speak the same language? Yes and no. But until 1850, they did neither speak nor write the same language. What Croats call "Croatian" is not either of the two Croatian indiginous languages -- the cakavian (pron. chakavian) and the kajkavian, of which the former is the only true Croatian tongue [kajkavian is a derivative of Slovenian and is encountered only in regions bordering with Slovenia].
While all Serbs speak shtokavian (or shtokavski), only a portion of people who identify themselves as Croats speak shtokavian (Serbian). Those Croats living predominantly in western Herzegovina, which was forced to accept Catholicism under Maria Theresa, typically have last names that sound Serbian -- Miloshevich being one of them, and their national instrument is gusle a one-string device originating in Montenegro (as do Miloshevichs). Interestingly, their dialect also contains sound specific for Montenegrin speech. Of course, the converts came from Orthodox ranks wo -- through equating Catholicism with Croatian national identity -- come to identify as such.
Likewise, in Slavonia, the shtokavski speaking "shoktsi" (sokci) come from a line of forced converts to RC. Their name comes from the old Serbian word for hand (shoka) which in the modern language means a clenched hand or fist (shaka). People could tell who were "shoktsi" by the way they crossed -- with the whole hand (shoka) instead of with the three fingers held together as one (the Orthodox manner).
Thus it are clear historical, linguistic, cultural (rleigious) and demographic dicatons that all shtokavski-speaking people in the Balkans were Serbs, just as it is clear from the decrees of Bosnian princes and kings that they were rulers of Serbs (and only Serbs, the only nation mentioned by name "Serblye"), while also in control of areas of the Dalmacia and Croatia.
Croatian documents from early Christianity were written in Old Church Slavonic for a long time, but secular documents were written strictly in cakavski -- a language that has a pleasant character and well suited for poetry, but is utterly underdeveloped for prose, and is close but unintelligible to shtokavski-speaking Serbs. Because of this, and because they were under Hungarian and Austrian rulers, Croatians used either Hungarian, German or Latin as the official written language for centuries. for example, at the beginning of the 19th century, the Croatian diet used exclusively Latin, and only German-language books could be bought in Zagreb, Croatia's capital then known under the name Agram).
In 1850, with the help of Austrian Court, and with clearly delineated political reasons having to do with weakining Russian influence in the Balkans, a naive and foolish Serb self-styled language reformer by the name of Vuk Stefanivich-Karadzhich, together with a Croatian representatives singed, without any authority or mandate, on behalf of their nations a Language Agreement which established shtokavian, the living spoken language of Serbs, as the linguistic norm for the common literally language of Serbs and Croats, written accoridng to the alphabetical rules pushed by Vuk and written either in Cyrillic or Roman alphabets.
Thus, Croats accepted Serbian as their linguistic standard and under the guise of "Illyrian" unity and pan-Yugoslav (South Slav) idealism called it "Croatian." This would be equivalent to saying that "American" is a language distinct from English, and that it is American, not English, for no reason other than Americans use it.
Just for the record.
" Just for the record."
Thanks. I'll give Bogdan a history lesson.