The prologue to John's gospel has the following line, which perhaps may be understood as a kind of summary of the effects of Christ's advent into the world:
The light shines in the darkness, and the darkness did not overcome it (John 1:5).
There are a few interesting things to note here. In the first place, consider the radically asymmetric difference between light and darkness. Light is the more substantial being, we might say, whereas darkness is purely privation and absence: darkness is just the absence of light in some place, whereas light is the true being with a substance of its own. Throughout the ages, Christian theologians have understood the relation between good and evil, righteousness and sin, etc. in similar terms: the good is substantial; evil is just a privation of good in some way or other. For this reason, too, God is not understood to be the cause of evil, because he is the cause of what exists and evil strictly speaking lacks existence of its own. It is parasitic on the good, rather than having substantial existence on its own terms.
The asymmetry in the relation between good and evil, light and darkness, means that one of the two is specially privileged and powerful over the other. Because evil is merely a privation of the good, therefore good is always superior and more powerful to evil. Likewise, because darkness is merely a privation of light, so also a lamp in a dark room overcomes the darkness effortlessly (even if not completely, depending on the strength of the light). This is why John says that the darkness has not overcome the light: because of the sort of thing darkness is, in principle the light always remains substantial, and darkness always a privation; a privation cannot cause anything, but only substantial being, and so the darkness cannot overcome the light. It would be absurd to think that a candle goes out because the darkness around it suppresses it! On the contrary, only if it runs out of oxygen or whatever else it needs to stay lit will it go out.
John is therefore saying that Christ, who is the Logos and the true light of all men (cf. v. 4), was not overcome by all the darkness which is in the world. In principle he could not be, since darkness cannot overcome light. Even though he was killed and crucified unjustly, he was still not conquered by the world but on the contrary, he says that he has overcome the world -- and that because he did not compromise the truth. This is a radical thought: true victory is not staying alive at any cost, but remaining faithful to the testimony of the truth even if others should kill you for it!
Of course, darkness by itself cannot make a candle blow out. But other things can -- the wind, a lack of fuel, etc. Our candles don't burn by their own power, we might say. Christ was not overcome by the darkness in the world, however, because he is that Light from which all else light derives. He has the power of life and light within himself, and if anyone else has light or life, it ultimately derives from the activity of Christ the Logos. If we do not wish to be overcome, then, we have to stay close to the Logos, in communion and fellowship with him, to draw from his life and his light in the darkness of this world:
I am the vine, you are the branches. Those who abide in me and I in them bear much fruit, because apart from me you can do nothing (John 15:5).