But there is no clear attestation for such a meaning as "among" or "in the midst" for the adverb
{entos} in any ancient Greek source. It is indisputable that "within" is the ordinary meaning, and the immediate context here also seems to favor this meaning. Here Christ is obviously contrasting the outward appearance (... "with observation," v. 20) with the inner spiritual reality of God’s rule. It was understood thus by the translators of all the ancient versions, and by all the Church fathers. Moreover, as S.C. Carpenter explains, "For 'among' S. Luke would have said ...
{en mesos--Greek doesn't show up well here}, which occurs seven times in his Gospel (see especially xxii. 27) and four times in Acts."
The circumstance that in Luke’s narrative these words are addressed to unbelieving Jews does not make any difference, because as Olshausen says, "The expression
{entos hymon} does not make the Pharisees members of the kingdom of God, but only sets before them the possibility of their being received into it, inasmuch as an internal and spiritual manifestation is made its universal criterion." (From first article/link below)