Acts of the Apostles 1:23-26
We dont hear much about Mattheus after this point. I've always believed that Paul replaced Judas and that he will be named after the pearly gates of Revelation.
Yes, St. Matthias replaced Judas Iscariot.
St. Paul was never one of the Twelve.
The Acts of the Apostles isn't meant to be exhaustive. You'll note that the Acts don't talk about most of the Twelve Apostles. For example, we don't hear about St. Simon the Zealot, or St. Thaddeus, or St. Matthew, or St. Thomas, et al.
But just because the Acts doesn't talk about them and what they were doing, doesn't mean they weren't active in their apostolic work--they were.
It's just that most of what we know about what they did (and the same is true with Matthias) comes to us not recorded in Scripture, through general Church tradition--and, admittedly, sometimes legends, and also occasionally with conflicting accounts making it sometimes difficult to parse.
Meaning that Matthias the Apostle was active and fulfilling his apostolic vocation--we just don't have any record of it in the Acts of the Apostles or anywhere else in the New Testament.
It can be easy to forget that there is real history, real events and persons and things going on, the New Testament records some of it for us, but we don't get anything exhaustive. Even as we read the epistles of Paul, we are really prying in on correspondence between Paul and his audience, but we are also only getting one side of that correspondence, Paul's. Paul talks about people and things going on that his audience knew very well, but which we don't. For example, we know that 1 Corinthians wasn't the first letter Paul wrote to the Corinthians, he speaks of an earlier letter--one which is now lost to us (1 Corinthians 5:9). And we know that members of the Corinthian church were in correspondence with Paul, as Paul even mentions finding out about things going on in Corinth from "Chloe's [household]" (1 Corinthians 1:11).
Understanding that the New Testament is a slice of the things happening in the apostolic period is important, it grounds the New Testament in history.
-CryptoLutheran