I have been with a girl and I am intending to marry her. The only thing is that she is from Sikh religion. It is stated that a Muslim can only marry among the people of the book (Torah, Bible and Qur'an of course). Because they believe in One God. Sikhism came after Islam so there wasn't any restrictions on the marriage between a Sikh and Muslim. They believe in one god also. I will raise my children Muslims, but I want to know if I will be doing a sin by marrying her.
Non-Muslims totally are of two kinds:
The first one consists of People of the Book
the second one consists of Not People of the Book.
In accordance with the view of all Shiite jurists, a Moslem man cannot get married (neither temporarily nor permanently) to a non-Moslem woman who isn't of the People of the Book
The significant point is that you ought to pay attention if it (Sikh) is really considered as "people of the book", if not, as a rule you cannot Marry her and it could be considered as a big sin.
No. See this, please:
Allah says (interpretation of the meaning): “And do not marry Al- Mushrikaat (idolatresses) till they believe (worship Allaah Alone). And indeed a slave woman who believes is better than a (free) Mushrikah (idolatress), even though she pleases you. And give not (your daughters) in marriage to Al‑Mushrikoon till they believe (in Allaah Alone) and verily, a believing slave is better than a (free) Mushrik (idolater), even though he pleases you. Those (Al-Mushrikoon) invite you to the Fire, but Allaah invites (you) to Paradise and forgiveness by His Leave, and makes His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) clear to mankind that they may remember” [al-Baqarah 2:221]