Mankind is a single nation. So Allah raised prophets as bearers of good news and as warners. . ."(The Holy Quran 2:213)
“And those who believe in Allah and His messengers and make no distinction between any of them (in belief), to them He will grant their rewards.”] (4:152)
“And certainly We raised in every nation a messenger, saying: Serve Allah and shun the devil.” (16:36)
stealing is when you take some thing that is not yours; most of the religions before being corrupted were once islam and some religious scriptures still do contain things what Allah Almighty deemed fit to keep in their original form; In which we muslims believe because they match with the teachings of quran and sunnah and that is how we know that they are true.
Ilm is momins meeras; he should collect it from where ever he can. ( Mafoom of a Hadith)