The Spirit's ministry to people in Old Testament times was not the same as it has been since the Day of Pentecost. Whatever it was, the Lord made it quite clear it would be different after Pentecost. This indicates both that the Spirit was at work then and that His work would take on a different character after Pentecost. While there is an alternative reading of the present tense in the second clause, i.e., is in you, most commentators prefer the future tense. This, of course, delineates the contrast between the ministry of the Spirit at the time the Lord spoke these words and the future ministry after Pentecost.
Many commentators simply seem to be unaware of any distinction being made here.
F. Godet's comment is to the point. Hence we must be careful neither to read with the Vulgate, menei in the future, He shall dwell in the first proposition, nor with some Alexandrines, esti , is, in the second. With this contrast in mind, we need to try to delineate and systematize what the Spirit did for people in Old Testament times. Three words seem to explain the Spirit's ministry to people in the Old Testament. He was in certain ones.