And do not marry Al-Mushrikât (idolatresses, etc.) till they believe (worship Allâh Alone). And indeed a slave woman who believes is better than a (free) Mushrikah (idolatress, etc.), even though she pleases you. And give not (your daughters) in marriage to Al-Mushrikûn^[] till they believe (in Allâh Alone) and verily, a believing slave is better than a (free) Mushrik (idolater, etc.), even though he pleases you. Those (Al-Mushrikûn) invite you to the Fire, but Allâh invites (you) to Paradise and Forgiveness by His Leave, and makes His Ayât (proofs, evidences, verses, lessons, signs, revelations, etc.) clear to mankind that they may remember. (2:221)

In the condition of someone who is living in a place where there is no Muslim slave lady available for marriage at all. Then the lady who is from the people of scripture, can be married, in order for one not to be involved in illegal sexual intercourse. Under the condition that she is a believer of the oneness of Allah and she does not worship any kind of statues (Jesus, Moses, etc.), and does not celebrate any kind of occasions, there are some who call themselves Muslim, but involve in non-Muslim activities. The oneness of Allah is very important, and her unconditional acceptance of Islam, because this is a matter of one's generations, after marriage. Muslim who is not a practicing person, and marries the people of the book, and puts all of the conditions, that she has to accept Islam, but he himself is not a Muslim if he does not practice, he is a bad example in front of his wife. 

...(Lawful to you in marriage) are chaste women from the believers and chaste women from those who were given the Scripture (Jews and Christians) before your time, when you have given their due Mahr (bridal money given by the husband to his wife at the time of marriage), desiring chastity (i.e. taking them in legal wedlock) not committing illegal sexual intercourse, nor taking them as girl-friends. And whosoever disbelieves in the Oneness of Allâh and in all the other Articles of Faith [i.e. His (Allâh's), Angels, His Holy Books, His Messengers, the Day of Resurrection and Al­Qadar (Divine Preordainments)], then fruitless is his work, and in the Hereafter he will be among the losers. (5:5)