Justin Martyr finished telling his audience that the Christians most certainly are not atheists. They just believe in one God and he's not their gods.
He moves on to say that the pagans assume that the Christians look for a human kingdom. They have no concept of a world beyond this one. Christians long for the kingdom that is with God. If their kingdom was for this world, then they would deny Christ to keep from being slain. That is not the case. They gladly suffer death to see the glorious kingdom.
"What sober-minded man, then, will not acknowledge that we are not atheists, worshiping, as we do the Maker of the universe, and declaring, as we have been taught that He has no need of streams of blood and libations and incense...We reasonably worship him."
The pagans may make human sacrifices and have wild parties, but the Christians do not mutilate themselves, get drunk, and they control their urges. While the pagans love only their neighbors and friends, the Christians pray for their enemies and love them.
Justin makes many contrasts using the Sermon on the Mount. I suppose the rest of the world would think that alien since they do not have the Holy Spirit to guide them.