That is an excellent question, and we do our best to answer it in our book, Don’t Blame God: A Biblical Answer to the Problem of Evil, Sin, and Suffering.
The handful of references to Satan in the Old Testament (see Gen. 3:1, Job 1:6-13; 2:1-7; Isa. 14:12-20, and Ezek. 28:1-19) did not fully reveal the being who Jesus revealed in the New Testament. Looking back in light of the New Testament, we understand these verses as referring to him, but they were sufficiently cryptic so as to conceal much information about him. They recognized the existence of a spirit realm, and good and evil spirits, but had no idea of the two spiritual kingdoms at war with one another.
The basic reason why God did not reveal the Devil to people in Old Testament times is that they were unprepared to deal with such a powerful spiritual being. They were not spiritually equipped to combat an invisible enemy with incredible supernatural ability who was committed to their destruction. Given that the vast majority of them had no spiritual weapons, the loving thing for God to do was to hide the Devil’s existence from them. To His credit, through the Old Testament God just “took the rap,” as it were, for evil by saying that if His people obeyed, He would bless them, and if they did not obey, He would afflict them. The latter is language that is explained in our book, Don’t Blame God! It is absolutely critical that Christians understand the Old Testament in light of the New Testament. God is love, and He will not do harm to His people.
Prior to the Day of Pentecost (Acts 2:1and following), only a few select people had the gift of holy spirit. One of the purposes of Jesus’ coming was to destroy the Devil’s work (1 John 3:8). On the Day of Pentecost Jesus poured out the gift of holy spirit, which inherently gives us spiritual power (Luke 10:19; John 7:39; Acts 1:8; etc.). Unlike the Old Testament, God has now revealed that “our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms” (Eph. 6:12). And further, unlike the Old Testament believers, we now have both spiritual power and authority to stand against the schemes of the Devil (Eph. 6:10 and 11).