The texts pull in two directions and the scholarship formed camps around each. Some sayings present the kingdom as imminent and future: this generation will not pass away, some standing here will not taste death, the disciples are to pray for it to come. Others present it as arrived: if I cast out demons by the finger of God, then the kingdom of God has come upon you.
Albert Schweitzer, at the start of the twentieth century, argued for the first without softening it. On his reading Jesus expected the end within a very short time, acted on that expectation, and was mistaken, and the whole of the earliest Christian movement is an adjustment to a delay. The position is usually called consistent or thoroughgoing eschatology, and its force lies in refusing to make a first century figure hold twentieth century opinions.
C. H. Dodd argued the opposite case a generation later. On his reading the decisive event has already occurred in the ministry of Jesus, and the future language is imagery for a reality now present, a position called realised eschatology. The synthesis usually credited to Joachim Jeremias and Oscar Cullmann holds both: the kingdom is inaugurated and not consummated, already and not yet.
Which reading is taken has practical consequences. A strongly present kingdom supports the social gospel and liberation theology in reading it as something to be worked for; a strongly future one supports a posture of waiting. Most churches now hold a version of the double position.