Spirit-baptism is the engrafting by God the Holy Ghost, in association with water-baptism, into the New Testament Visible Church at her birth on Pentecost Sunday. It still recurs thereafter, in association with water baptism, whenever new Members are thereby engrafted into that Church Visible. Symbolically, it signifies and seals the regeneration of God's elect. See Westminster Confession of Faith 25:2b & 28:1b and Westminster Larger Catechism 62w, 161s, 162a, 165l, & 167pqy.
In Old Testament times, no Spirit-endued or Spirit-imbued person was thereby "baptized in the Spirit." All Old Testament predictions of Spirit-outpouring, were fulfilled only at Christ's own baptism — as consummated on Pentecost Sunday.