I have been wondering if Theophilus was a Sadducee. Here are my clues:
1. Sadducees deny the resurrection, angels, and spirits (or spiritual existence) (cf. Acts 23.6ff.; Josephus).
2. Luke makes much of angels (Luke 1.11, 13, 18-19, 26, 30, 34-35, 38; 2.9-10, 13, 15, 21; 4.10; 9.26; 12.8-9; 15.10; 16.22; 20.36; 22.43; 24.23; Acts 5.19; 6.15; 7.30, 35, 38, 53; 8.26; 10.3, 7, 22; 11.13; 12.7-11, 15, 23; 23.8-9; 27.23).
3. Luke has an extended resurrection narrative, including an ascension into heaven as realm of spiritual existence (Luke 24.1-52; Acts 1.1ff.).
4. Luke most often names the high priest(s) in question [I need to find references to all mentions by name of high priests]. However, in 5.17ff (cf. also 4.1), he simply refers to a high priest without naming him. In both instances, that high priest is said to be of the party of the Sadducees. Why not name him here?
Can it be demonstrated with relative certainty that Acts 5.17 (and 4.1ff?) constitutes events post-37AD, when Theophilus was serving as HP? It comes on the heels of a general summary statement with no indication of time span (5.12-16), common in the early chapters of Acts (2.42-47; 4.32ff; 5.12-16; 5.42). There was probably a 14 year gap between Acts 9 and 13, between Paul's conversion and his ministry beginning (according to Galatians). What can also say with relative certainty (with the majority of scholars) that Acts 8, the "Samaritan Pentecost", must have been a number of years after the Pentecost event of Acts 2. Can we know what time Acts 6 and 7 depicts, it being a single event (Stephen's sermon and stoning)? If so, then maybe we can come close to determining when Acts 5 took place, and whether or not Theophilus was the unusually-anonymous high priest of 5.17, thus making him a Sadducee, according to the text.