Floyd Parker, «The Terms "Angel" and "Spirit" in Acts 23,8», Vol. 84 (2003) 344-365
In any discussion of the Sadducees, there will always remain a certain amount of doubt due to the paucity of sources about them. Based on what data has survived, the older theory that the Sadducees rejected the extravagant beliefs about angels and spirits provides the most convincing solution to the problem of Acts 23,8. The Sadducees’ reasons for rejecting these views were twofold: 1) angels were integrated into the apocalyptic world view that they rejected; and 2) angels often served as God’s servants to administer predestination or providence. Thus, when Paul claimed that a heavenly being had appeared to him in a manner and with a message that appeared to be predestinarian in nature, the Sadducees were unwilling to entertain the idea that an angel or spirit had appeared to him. Certainly new theories will arise in an attempt to grapple with this issue, but to re-appropriate the words of Jesus in Luke 5,39, "the old is good enough".
Another option is that "angel" and "spirit" are used to distinguish genera (e.g., Cherubim, Seraphim, and Ophanim) or rank (e.g., archangel and angel) of heavenly beings72. If so, the Sadducees might have rejected them on the same grounds as angels, that is, for their role in the oversight of predestination.
Finally, "spirit" could refer to the Holy Spirit73. However, this option seems somewhat less likely, due to the fact that Luke often qualifies this use of Spirit with words like "holy"74. There are some uses of "spirit", in the sense of "Holy Spirit", without qualifiers. Even then, other words in these contexts frequently make it clear that the Holy Spirit was intended by the author75.
3. Criterion Three: Relevance to the Context of Acts 23
With these facts in mind, it is time to turn to the context of Acts in order to see why Paul’s description of his religious experiences would have been objectionable to the Sadducees. The primary objection here seems to be that an angel or spirit "spoke" (e)la/lhsen) to Paul76. Why, then, would the Sadducees have had an objection to a spirit or angel speaking to Paul? I propose that they perceived both the event of revelation to Paul and the content of that revelation via an angel or a spirit to be at odds with their view on fate or providence.