Although I am a dispensationalist, I do not label myself as either Acts 2, Mid Acts, or an Acts 28 Dispensationalist. For me, I see God's revelation progressing all the way through the book of Acts. I do not see a good place to "park" anywhere in the book of Acts as far as a "doctrinal boundary" is concerned. My position is all scripture is profitable for doctrine as 2 Tim 3:16 states. A "workman" has to rightly divide all scripture, and these divisions are not straight lines like the Acts 2, Mid Acts, or Acts 28ers would have you believe. Some of the Acts 28ers believe that Romans 16:25 was added to the book of Romans sometime after Acts 28, because they do not believe the "mystery" was revealed until Acts 28 or later. The Acts 2 and Mid Acts brethren argue that Romans 16:25 was written at the same time as the rest of the book of Romans (i.e. before Acts 28), since they believe Paul was preaching the mystery shortly after his conversion. I can not prove that Romans 16:25 was written at the same time as the rest of the book of Romans, and I can not prove that Romans 16:25 was added after Acts 28. For me it does not matter, because although the fact of the "mystery" is manifest or revealed at the time of the writing of Romans 16:25, the details of the mystery are not spelled out in Romans. It is my belief, based on Romans 1:11 that Paul gave the details of the mystery (i.e. imparted a spiritual gift) sometime after he got to Rome. Paul says this "spiritual gift" (the mystery and its details) would end up establishing the believers at Rome. Romans 1:11 states, "For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established..." In fact Romans 1:11 is a good cross reference to Romans 16:25 where Paul lists several things that would "stablish" the believers at Rome, and one of those things is said to be the "mystery." When Paul writes Romans 15 Paul indicates that the kingdom salvation for the Gentiles is still in view. Romans 15:8-12 states:
[8] Now I say that Jesus Christ was a minister of the
circumcision for the truth of God, to confirm the promises made unto the
fathers:
[9] And that the Gentiles might glorify God for his
mercy; as it is written, For this cause I will confess to thee among the
Gentiles, and sing unto thy name.
[10] And again he saith, Rejoice, ye Gentiles, with his people.
[11] And again, Praise the Lord, all ye Gentiles; and laud him, all ye people.
[12]
And again, Esaias saith, There shall be a root of Jesse, and he that
shall rise to reign over the Gentiles; in him shall the Gentiles trust.
Note that Paul makes it clear that Christ's earthly ministry had a twofold purpose. First, it was to minister to the circumcision (Israel) the truth of God. Second, after Israel accepted the truth, then Israel would be a witness to the Gentiles of God's truth resulting in the Gentiles accepting God's truth. Paul does not indicate anywhere in Romans 15 that anything has changed in God's plan to reach Israel, and then to bless the Gentiles. In fact Paul implies that his own ministry is in accord with what Isaiah said about the Gentiles in that he quotes part of Isaiah 52:15 in Romans 15:20-21.
Most of the Acts 2 and Mid Acts brethren try to make Paul a minister preaching the mystery, as defined in Ephesians and Colossians, during all of the Acts period. However, once you understand what the mystery is, you have to do a lot of twisting and turning to get Paul preaching the mystery before the end of the book of Acts. In fact in Romans 15:16 Paul actually states what his ministry consisted of during Acts: "That I should be the minister of Jesus Christ to the Gentiles, ministering the gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Ghost." Note that Paul says he is ministering the "gospel of God" to these Gentiles, and this Gospel of God was not a mystery (see Romans 1:1-3). The gospel of God is according prophecy and not mystery! Paul goes on to say in Romans 15:27 that during Acts the Gentiles were partaking of the spiritual things of Israel, and this certainly could not refer to the mystery program Paul speaks of in Ephesians and Colossians. The mystery made Gentiles equal with the Jews. Consider how the Gentiles are called "fellowcitizens" (Eph 2:19) and "fellowheirs" (Eph 3:6) under the mystery program. By definition a "fellow" is an equal, and it is obvious that the Gentiles during the Acts period were not equal. The gospel, as late as the book of Romans, was still to the Jew first (Rom 1:16). Under the mystery program the gospel is to all men alike. Romans even states that the Jews still had an advantage during the Acts period (Romans 3:1). It is true that Paul did have a distinct gospel during the book of Acts, but Paul does not equate it with the "mystery." He lists "my gospel" as a separate item from the "mystery" in Romans 16:25. "Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began..." Paul's gospel of the uncircumcision was a grace message to the Gentiles in which they did not have to observe the law of Moses (Acts 15:5), but they did have to be observe "necessary thing" in order not to offend the Jews (Acts 15:28-29 & Acts 16:4). Under the mystery the gospel is still of grace, but the Gentiles no longer have to worry about offending the Jews in meat or drink (Colossians 2:16-17).
Based on my study of Acts and Paul's epistles, it is my belief that a better way to divide Paul's early epistles and Acts with his later epistles, is by considering the statements in 1 Corinthians 9:17 and Colossians 1:25-26. In Acts and Paul's early epistles written before Acts 28, I think we can say based on 1 Corinthians 9:17 "a dispensation of the gospel is committed unto me" (i.e. Paul). Paul preached a gospel message of grace to the Jew first and then to the Greek, with the idea that the Kingdom was still at hand if Israel repented. This would explain Paul still praying for Israel's national salvation in Romans 10:1. Since Paul believed Israel would repent and the Kingdom would be setup in the first century, he told the believers at Rome God would bruise Satan "shortly" (Rom 16:20). With Israel's final rejection of God's message in Acts 28, the kingdom offer is withdrawn. At some point afterward Paul imparts the details of the mystery. The "dispensation of the gospel" ends with Israel being set aside in judicial blindness at Acts 28. The details of another dispensation are given by Paul in Ephesians and Colossians. The gospel is still one of grace, but it is no longer to the Jew first,the earthly kingdom for Israel is no longer viewed as imminent, and the Jew no longer has an advantage. This dispensation is called "the mystery" according to Colossians 1:25-26:
[25] Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God;
[26] Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
This dispensation is also called the dispensation of the grace of God according to Ephesians 3:2.
The existence of a "mystery" was revealed at the time Romans 16:25 was written. The time of writing really does not matter, because the details of the mystery were not given and practiced by Paul until Ephesians and Colossians. Paul's ministry covers two dispensations, the dispensation of the gospel during the book of Acts, and the dispensation of the mystery or the dispensation of the grace of God after Acts 28.