I live in the US and want to give my child an American name. A name I really like is "Thomas", which, of course, is a Christian name. I thought it could be justified by Thomas, in the Bible, is a friend of Issa. But do we in Islam believe Thomas as Issa's friend? Thus I would like to know if it would be haraam to name my child "Thomas"?

Thomas is originally from Aramaic and means "twin". The general rule with baby names in Islam is that all names are acceptable unless they have a bad meaning. Since the meaning of Thomas is not bad, it is an acceptable name for a Muslim baby. Scholars generally do not recommend using names inspired by other religions, but this is only a preference and Muslims are not required to abide by it. The Prophet, peace and blessings of Allah upon him, had a wife whose name was Marya, which is the Coptic Christians version of the name Maryam/Mary. Even though the name was from a Christian background, the Prophet approved of it and did not ask her to change it.

For cultural reasons some Muslims may disapprove of using Western/Christian names, so you will have to be prepared to deal with their disapproval and having to explain to them that it is an acceptable name. From a religious standpoint it is a perfectly halal name.