Gregory of Nyssa
Catena Aurea · Lukasevangelium 17
Lk 17, Verses 20–21English original
And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation: Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you.
(lib. de prop. sec. Deum.) Or, perhaps, the kingdom of God being within us, means that joy that is implanted in our hearts by the Holy Spirit. For that is, as it were, the image and pledge of the everlasting joy with which in the world to come the souls of the Saints rejoice.