An apostle (ἀπόστολος) is a messenger who is sent out on a specific mission; however, unlike a regular messenger who delivers a message, an apostle is sent with a specific task to perform while giving the message.
Christ sent out the 12 apostles to the Jews during His earthly ministry to proclaim the message of the kingdom of the heavens, Matthew 10:5-42. They were not to take any provisions with them for their journey or go into any city of the Gentiles nor to the Samaritans, for they were sent only to the lost sheep of Israel. Herod sent forth men with a specific mission to slay all the children in Bethlehem who were two years old or younger after the Magi left the region, having been warned by a messenger, failing to report where they found the King of the Jews, Matthew 2:16.
In a single act of expressing love towards the inhabited world, God the Father sent the Son to be the Savior of the world, 1 John 4:14. He sent Him on a specific mission so that through Him we should have life, 1 John 4:9. In this we know love, for God sent His Son to be a propitiation for our sins, 1 John 4:10.
When Christ ascended, He gave gifts to men for the edification of the Church to bring us to a oneness of the faith so that we are no longer tossed around like inarticulate babblers by every wind of teaching by the trickery of men, Ephesians 5:8. Of these gifts were the Apostles, Ephesians 5:11. Twelve of them to the Church. These twelve laid down the foundation of the Church, built upon Christ, giving us the entire revelation concerning the whole council of God. Christ instructed them before His final ascension that as they were caused to go, they were to make disciples of all nations, teaching them to observe all that Christ had commanded them, Matthew 28:18-20.
In the upper room, before the disciples were aware that Paul would be the twelfth Apostle to the Church, in determining to replace Judas, they gave the requirements for a disciple who could be counted among the twelve. They had to be with them for the entire time the Lord Jesus went in and out, from the beginning of John to the day He was taken up; therefore, seeing the resurrected Christ, Acts 1:21-22. As one of the Pharisees, Paul knew of the immersion of John and would have known about Christ. Paul is the last of those who saw the resurrected Christ and considered himself to be one who was born out of time regarding becoming an Apostle to the Church, 1 Corinthians 15:1-9.
The Apostles to the Church are the twelve men, eleven of whom walked with Christ during His earthly ministry, that were given to the Church by Christ when He ascended on High, Ephesians 4:11. There are no additional Apostles added to the Church today, for these twelve make up the foundation, Revelation 21:14. And anyone claiming to have seen Jesus today is lying, for when we see Him we will be like Him, 1 John 3:2, and none of us will be in our current condition before all of us receive our full redemption, the resurrection of our bodies. Not even those who have died in Christ see Him before we all share in that moment, 1 Thessalonians 4:16-17.
Therefore, the title of an Apostle in the Church is reserved for only the original twelve apostles who were sent specifically to proclaim the Gospel of the Christ. Eleven of whom focused on Israel and Paul, who is the apostle to the Gentiles, Acts 18:6.