As others have suggested, the Holy Spirit was certainly operative, albeit in a different, limited way, prior to the advent of Christ. At our first creation, the Holy Spirit was fully accessible to man, but, as St Cyril of Alexandria taught, the fullness of the indwelling Grace of the Holy Spirit was lost when He departed from man as a consequence of the fall.
St Cyril identifies the Baptism of Christ in the Jordan as the decisive point at which the Spirit signifies His return to the human race. In responding to an Arian argument that in receiving the Holy Spirit at His baptism Christ demonstrated that He did not, therefore, naturally possess Him and was in some way in need of Him, St Cyril explains that Christ received the Spirit only so that He, "might preserve it in our nature and [so that] He who knew no sin might again inroot in us the grace which had left us…by reason of sin…" (Commentary on the Gospel of St John).