What happens to people who had never heard of Christianity

Jesus says that the judgement will be according to 'how much has been given' to someone.

Luke 12:48 To whoever much is given, of him will much be required; and to whom much was entrusted, of him more will be asked.

Thus people who never knew much will be treated a lot milder than those who knew a lot.

Matthew 11:22 But I tell you, it will be more tolerable for Tyre and Sidon on the day of judgment than for you. 23 You, Capernaum, who are exalted to Heaven, you will go down to Hades. For if the mighty works had been done in Sodom which were done in you, it would have remained until this day. 24 But I tell you that it will be more tolerable for the land of Sodom, on the day of judgment, than for you.”

Paul says that people who never heard the law of God either may be accused or excused by their conscience or thoughts when they are judged by God.

Romans 2:11 For there is no partiality with God. 12 For as many as have sinned without law will also perish without the law. As many as have sinned under the law will be judged by the law. 13 For it isn’t the hearers of the law who are righteous before God, but the doers of the law will be justified 14 (for when Gentiles who don’t have the law do by nature the things of the law, these, not having the law, are a law to themselves, 15 in that they show the work of the law written in their hearts, their conscience testifying with them, and their thoughts among themselves accusing or else excusing them) 16 in the day when God will judge the secrets of men, according to my Gospel, by Jesus Christ.

Isaiah says that at the end of times God will gather people among those who had never heard of him, who He still considers to be 'brothers of Israel'.

Isaiah 66:18 For I know their works and their thoughts: the time comes, that I will gather all nations and languages; and they shall come, and shall see my glory. 19 I will set a sign among them, and I will send such as escape of them to the nations, to Tarshish, Pul, and Lud, who draw the bow, to Tubal and Javan, to the islands afar off, who have not heard my fame, neither have seen my glory; and they shall declare my glory among the nations. 20 They shall bring all your brothers out of all the nations for an offering to Yahweh, on horses, and in chariots, and in litters, and on mules, and on dromedaries, to my holy mountain Jerusalem, says Yahweh, as the children of Israel bring their offering in a clean vessel into the house of Yahweh. 21 Of them also will I take for priests and for Levites, says Yahweh. 

Some of them will even become priests, meaning they get important positions.

As a summary we should conclude that God is a just God and will decide in a righteous way about everyone.