The Kingdom of God is primarily an eschatological concept. The fundamental idea of the Kingdom in Scripture is not that of a restored theocratic kingdom of God in Christ—which is essentially a kingdom of Israel—, as the Premillenarians claim; neither is it a new social condition, pervaded by the Spirit of Christ, and realized by man through such external means as good laws, civilization, education, social reforms, and so on, as the Modernists would have us believe. The primary idea of the Kingdom of God in Scripture is that of the rule of God established and acknowledged in the hearts of sinners by the powerful regenerating influence of the Holy Spirit, insuring them of the inestimable blessings of salvation,—a rule that is realized in principle on earth, but will not reach its culmination until the visible and glorious return of Jesus Christ. The present realization of it is spiritual and invisible. Jesus took hold of this eschatological concept and made it prominent in His teachings. He clearly taught the present spiritual realization and the universal character of the Kingdom. Moreover, He Himself effected that realization in a measure formerly unknown and greatly increased the present blessings of the Kingdom. At the same time He held out the blessed hope of the future appearance of that Kingdom in external glory and with the perfect blessings of salvation.
Louis Berkhof, Systematic Theology (Grand Rapids, MI: Wm. B. Eerdmans publishing co., 1938), 568.