People often question what there is in Christianity that is not in other religions since many of the moral teachings are very similar and some even pre-date the birth of Christ. This is missing the point. What there is in Christianity that is nowhere else is Christ himself. The actual person of Christ who through his crucifixion and resurrection transformed the whole world, even those parts of it that had no conscious knowledge of him. He changed the spiritual goal fundamentally, opening up the Kingdom of Heaven to all who accepted him whereas before there were only states analogous to Limbo, Hades, Sheol, Bardo and the like for most souls with Nirvana as a possibility for those who were able to detach themselves completely from identification with their created selves. But the new reality of Heaven (new as far as human beings were concerned) meant that the fallen self could now be redeemed and transformed into a being of light. For Heaven involves the union of spirit and matter instead of their separation. It's a marriage not a divorce.
Regarding teachings of love that may have existed before the time of Christ, I think we can see these as analogous to the twilight that appears at dawn just before the rising of the sun. Prior to the actual Incarnation the spirit of Christ will have begun to permeate the higher worlds, affecting all those able to respond to it on that level. Thus, the teachings of Christ will have begun to be established in the world. A similar thing happened after Christ's death and means that all spiritual forms in the world that were open to it, whether ostensibly Christian or not, will have begun to be influenced by what we can call the Universal or Cosmic Christ. These forms and those that followed them may not know Christ the earthly man but they will know his spiritual presence which explains how non-Christians can be saved. They don't have to know Jesus the human being if, through their religion, they can respond to Christ the spiritual being. Christianity is the religion in which Christ is most fully present but surely you did not think he could not manifest himself and be present as a spiritual force and influence in other religions? He is primarily a spiritual being and as such can be present everywhere to those who are open to him. Such people may not know him by the name Jesus Christ but they can know his spiritual presence and that is what really counts.
This is my belief. I know many Christians won't like it because it says that Christ is present in religions other than Christianity, even though he is most present in Christianity and those who are born in the West should certainly seek their spiritual nourishment at source. And non-Christians won't like it because it puts Christ at the head of the spiritual table. He is the host at the feast to which we are all invited as honoured guests. But consider the matter like this. If Christ is the Way, the Truth and the Life, and he is, then he is the sun towards which all life forms must turn their face if they are to receive spiritual light and warmth.