St Matthias most certainly is considered an Apostle by the Church. Where are you getting your assertion from?
St. Peter. No, really. Read what Peter says in Acts 1. He is calling for someone to fill the office vacated by Judas. Apostle was not and is not an office in the church. Peter, referring to Judas, says in Acts 1:20:
"For it is written in the book of Psalms, ‘Let his habitation become desolate, and let there be no one to live in it’; and ‘His **office **let another take.’ [Acts 1:20 RSV]
The KJV and the DR has this verse as:
Acts 1:20 - For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his
bishoprick let another take."
The word translated as **office **or
bishoprick is the Greek word
episkope which is translated in other NT writings (Paul’s Epistles) as ‘bishop’.
In Acts 1:20 Peter is actually quoting a single verse from each of two different Psalms. The first, “Let his habitation become desolate, and let there be no one to live in it” is a paraphase of verse 25 of Psalm 69 while the second, “…his bishoprick let another take” is verse 8 from Psalm 109. Peter is not doing what bible scholars warn against; that being joining two separate verses together out of context. What he is doing is referring to two different things about Judas. The first verse refers to Judas’ lineage, his Apostolic Succession. That is what the word habitation refers to. One’s habitation is where one lives. It is one’s house. Recall that Jesus and Joseph were both said to be of the house of David meaning they were descended from David. Peter says Judas’ house will be desolate and no one will live in it. That means no one will claim any lineage or succession [Apostolic Succession] from Judas and to this day no bishop ever has claimed succession from Judas.
The second quote from verse 8 of Psalm 109 refers to the office Judas vacated. That is his “bishoprick” or episcopacy. Someone else will be made bishop to fill the office just as others will fill the offices of bishop that the other Apostles will vacate upon their deaths too. So Matthias is a bishop. Nothing more. Peter did not call for another Apostle. In fact Luke makes this quite clear in verse 26 of Acts 1 where he writes:
"Acts 1:26 - And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.’
Notice before the election of Matthias there are eleven Apostles. After the election there are still only eleven Apostles. Luke says he was numbered with the eleven but if he were an Apostle then verse 26 would say there were again 12 Apostles not still eleven. Luke says Matthias was numbered with them but does that actually make him one of them?
Recall that Isaiah 53:12 in prophesising about Jesus says of Him:
" Therefore will I divide him a portion with the great, and he shall divide the spoil with the strong; because he hath poured out his soul unto death:
and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors.
So Jesus was numbered with trangressers (sinners). Does that make Jesus a sinner? No more so than being numbered with the Apostles makes one an Apostle [especially when the number of Apostles stays the same].
Matthias is a bishop, a missionary bishop to be sure but only a bishop and nothing more.