FK, what the Reformed doctrine holds is that when Ananias told St. Paul to "be baptized, and wash away thy sins, calling on the name of the Lord...." -- well, he really didn't mean it literally that way. :-)
Yeah, yeah, yeah ... :) I've never studied Acts 22:16 before, so this was something of a challenge. From what I found, it appears that this is a classic case of dueling scriptures (this scripture that apparently says that baptism washes away sin, versus all the other voluminous scriptures that say the work of Christ on the cross washed away our sins), and an interpretation must be made. It seems the central question to legitimizing the Reformed position is WHEN was Paul actually saved, or when did he become a believer? Was he saved on the road to Damascus, or was he saved when he was baptized at the house with Ananias? I think a decent case can be made for the former.
First, we know that the Gospel he preached he received directly from Christ, not from Ananias:
Gal. 1:11-12 : 11 I want you to know, brothers, that the gospel I preached is not something that man made up. 12 I did not receive it from any man, nor was I taught it; rather, I received it by revelation from Jesus Christ.
This means that for him to have become a believer at baptism, he would have had to discount what Jesus taught him personally, but then accepted it through Ananias at baptism. I somehow doubt that this is how it occurred. Christ was calling him and instructing him in person. Paul was evidently moved to use the word "Lord", and he apparently was sufficiently impressed to follow the command of Jesus to continue on his way into the city, even though he had just been struck blind.
Now, I know that we have different ideas of what salvation is as contrasted to the remission of sins. But, we have no evidence that Ananias stood in as Paul's proxy because Paul did not have faith at his baptism, so I hope we can agree that he had it by that time. I am assuming that adults must have faith to be baptized. That makes it important when Paul got faith.
One evidence that we have that Paul already believed well before he was baptized (i.e. while still blind) was three verses earlier, in 22:13. Here, Ananias calls Paul "Brother Saul".
1 Cor. 5:11 : But now I am writing you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a man do not even eat.
This indicates that in Paul's mind, a "brother", in this context, was a believer. Since there was no question of any sibling relationship between Paul and Ananias, it appears reasonable that the term may have been used in a similar way here.
The bottom line is that consistent with the Reformed position, if Paul accepted Christ in His presence, then he already had the Spirit then, and his sins were remitted then (from the human POV). If, OTOH, Paul rejected Christ while in His presence, as Christ was calling Paul to His ministry, UNTIL (or unto? :) Ananias talked him into believing, (because Jesus didn't/couldn't?), then verse 16 could mean that baptism actually washes away sins. I just don't see this as reasonable.
One counter to all of this could be to say: "Who cares when Paul believed? Even as a believer his sins were not remitted until baptism. This brings us into looking into where the greater weight of scripture is. Is it on the side of baptism remitting sins, or is it on the side of Christ remitting our sins?
The Reformed view is consistent with the majority of scriptures such as:
1 Cor. 15:3 : For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, ...
Gal. 1:4 : ... who gave himself for our sins to rescue us from the present evil age, according to the will of our God and Father, ...
1 Pet. 3:18 : For Christ died for sins once for all, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive by the Spirit, ...
In my view, all of these verses, and many more, go against the interpretation that baptism actually washes away sins. Therefore, I believe the greatest weight of authority, which for me is the Bible, says that it was Christ who washed away the sins of His elect, not anyone's baptism.