It's obvious you don't really know what christianity actually says. Faith is a gift from God given to the "believer" after regeneration. Eph 2:8 tells us...
8 For by grace you have been saved through faith; and that not of yourselves,
it is the gift of God;
As you see the bible teaches faith is a gift from God...and not of ourselves. As far as being exposed to the "basic concept of Christianity"...that's one way God gives an individual faith.
It seems as if Paul expected a response such as you just made when he answered your question with:
Romans 9:
14What shall we say then? Is there injustice on God’s part? By no means! 15For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.” 16So then it depends not on human will or exertion,b but on God, who has mercy.
God carries out His mercy in many ways. It is true, in some instances God exposes christianity to children. There are many instances where God exposes christianity to adults. Not every person receives mercy. In fact no one deserves mercy but rather justice.
You posted "Most people that I have come across view the good afterlife as the reward for keeping faith"...in the sense you present it, that being salvation, the people who say that are not presenting to you correct biblical doctrine.
here's why, the bible speaks of two judgements. The
White throne for those that have not received mercy, but rather justice...and the second judgement reserved for the believers who have received mercy. This judgement is called the
Judgement seat of Christ or the Bema Seat judgement. The Bema seat judgement doesn't concern ones salvation as they are already saved but rather judges the believer on how they served the Lord.
I hope this post helps and straightens out some of your misreprentations of Christianity.