Consider the following translations of Luke 17:21:
NIV:
21 nor will people say, ‘Here it is,’ or ‘There it is,’ because the kingdom of God is in your midst.”
NRSVUE:
21 nor will they say, ‘Look, here it is!’ or ‘There it is!’ For, in fact, the kingdom of God is among[b] you.”
CEB:
21 Nor will people say, ‘Look, here it is!’ or ‘There it is!’ Don’t you see? God’s kingdom is already among you.”
KJV:
21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you.
All things different translations say slightly different things about the Kingdom of God. It’s either “in the midst,” “among,” “already among,” or “within us.”
To say the Kingdom of God is already among us is to emphasize that Heaven to some extent has already been brought to Earth, first and foremost through Jesus Christ’s incarnation, but also continued in the transforming power of the Holy Spirit in the lives of those who are disciples of Jesus, made manifest through effects here and now in the material world, primarily in how we treat “the least of these” i.e. the poor, hungry, and downtrodden.
In contrast, to say that the Kingdom of God is within us speaks more to the mystical and contemplative traditions of Christian history, whereby an emphasis is placed on God as the ground of Consciousness itself. According to this conception, seeking the Kingdom of God is experiential. Instead of merely seeking propositional knowledge of God, the goal is, more fundamentally, to achieve salvation here and now through theosis, union with God, the process of sanctification, whereby we put on the mind of Christ and partake in divinity, usually achieved by means of following ascetic practices such as prayer, fasting, meditation, chanting, contemplative scripture reading, etc.
However, I think the best translation ought to be “the kingdom of God is both within and among us.” It is a both/and type of situation. The “among us” translation rightly emphasizes the way in which God’s Sovereign Will is spreading, like leaven, throughout the Earth in a concrete, material way, starting with Christ and continuing through his disciples and the Holy Spirit. “Among us” emphasizes the material reality and concreteness of “Thy kingdom come, thy will be done, on earth as it is in heaven.” It emphasizes our call to help the poor, hungry, and downtrodden, to give up our riches, luxury, and comfort to help the “least of things,” such that the last will be first.
However, the “within us” translation emphasizes that the Kingdom of God does not just come in the midst of the material world, but also within the inner life of our consciousness, transforming us, bringing us closer to union with God, sanctifying our minds, bringing wisdom, understanding, and inner peace. This is the path of all the great mystics and contemplatives who seek to not just have beliefs about God but to experience God, to recognize our inner divinity, to become Christlike in our consciousness, to transform our consciousness from egotistical selfishness to radical nonduality, recognizing our essential oneness in Christ.
We must then acknowledge that the reality of the Kingdom of God cuts across the inside/outside distinction. It is both an active concrete force in transforming the material Earth into a revolutionary kingdom whereby poverty is alleviated, but through the Holy Spirit and the wisdom of Christ, also a means to transform our inner life as well, the transform the world of thoughts, feelings, and imaginations such that we have peace and calmness in our minds, not just in our actions.
If we emphasize the “outside” and “amidst us” exclusively we miss the opportunity for encountering God through the wholeness of our being, and likewise we lose a sense of wholeness if we put too much emphasis on the “inner” at the expense of transforming the “outer.” A balance is needed that shifts dynamically according to what’s needed in the moment. Silence and sound complement each other in time. There is a time for quiet inner reflection and a time for outward engagement. Ideally, both the times of the reflection and the outward engagement are done with a whole-hearted love towards God, without any of the neurosis of ego that takes actions, inner or outer, and corrupts them with a layer of neurotic self-consciousness, corrupting our spirit and making us ineffective as disciples of Christ.