Karl Rahner was born in 1904 and died in March 1984. He was one of the most influential Catholic theologians of the 20th Century, in part because of his impact on the Second Vatican Council, which happened between 1962 and 1965 (now that's one long meeting), and which I don't know much about except that it was Very Significant and marked Important Changes in Catholicism. Rahner was also German, which is significant (in my mind) largely because it means he's really difficult to read. This is a shame because (as is the case with most 20th century German theologians who are difficult to read i.e. pretty much all of them) when you work out what he's saying, it's really quite interesting.
One of his better known ideas is that of "anonymous Christians", which I'll attempt to explain below, and which pops up later in various other theologians' works, including some of the liberation theologians (see below). Most of what I know about this comes from his Foundations of the Christian Faith, in case you're ever tempted to read it for yourself.
Rahner argues that because of the Incarnation, the two greatest commandments to love God and love your neighbour are united: love of neighbour becomes love of God. To fully accept humanity becomes to accept the Son of Man, because in him God has accepted humanity. To love your neighbour becomes fulfilment of the law, because God himself has become our neighbour.
For this, among other reasons, Rahner answers the question of other religions and their relationship to Christianity with the idea of anonymous Christianity. If we are to say that Jesus is the salvation of all people, he must be present in the history of all people. He says that Christ is present in non-Christians and therefore in non-Christian religions through the Holy Spirit, and that wherever there are faith, hope and love, these justify people because, whether people are aware of it or not, these three always refer to Jesus. Consequently, there are two extremes: conscious and explicit Christianity, and anonymous and implicit Christianity, and a whole range of shades of grey in between. Rahner argues that Christianity is essentially existential - fundamentally about what American evangelicals would call "a personal relationship with Jesus" rather than doctrinal assent, and so as love of neighbour can be the same as love of God, it's possible for people to have that personal relationship with Jesus without necessarily recognising or naming it as such.
This is an appealing idea in many ways: it allows the possibility of seeing truth in all religions, and of seeing God at work throughout the history of the world and not just in the history of the Jewish people and then the church. But it does have its problems: I can't help wondering how Rahner's "anonymous Christians" would feel about being told that, while they think they're following Buddha or Mohammed or Marx, they're actually following Jesus. I wonder how Rahner would feel if someone told him that, for all that he thought he was a Christian, he was actually an "anonymous Hindu"?