Just look at the old testament - there is no adversary
Have you actually ever bothered to read the bible?
The Hebrew scriptures ("Old Testament") deals mostly with God's dealings with ancient Israel (See Genesis 17:7, 8 and Exodus 19:3-6) and of the lineage that would lead to the promised seed first mentioned at Genesis 3:15 (See also Genesis 22:15-18; Galatians 3:16 and Matthew 1:1-17), however several times throughout the Hebrew scriptures the entity known as Satanas throughout the Greek scriptures ("New Testament") is certain present in the Hebrew scriptures (The word Satan itself is Hebrew). One particular example that comes to mind is Job 1:6-9 which mentions Satan several times. Given that all scripture is inspired (2nd Timothy 3:16) it is noteworthy that Revelation 12:9; in the Greek scriptures, points back to Genesis 3:1; in the Hebrew scriptures, identifying the original serpent in the Garden of Eden as Satan.
I could go on with more examples but I'm just making the case that God's chief adversary, Satan the Devil, is indeed present in the Hebrew scriptures.