The apostles were those sent out with the direct commission of Christ, and in the original and strict sense the office could not be transmitted, which is the root of a long argument in Christianity.
Witnesses and envoys
The word means one sent, and the apostles were the envoys commissioned by Christ himself, the inner group of the Twelve together with a few others such as Paul, charged with founding and governing the earliest church and bearing witness to the resurrection. In the strict sense the office was unrepeatable, since it required a direct commission from the risen Christ and, for the Twelve, having accompanied him, so the apostles could have no simple successors in the full sense of their office.
Succession and its disputes
From this arose the doctrine of apostolic succession, the claim that the authority of the apostles passed to the bishops they appointed and so continues in the episcopate, which the Catholic and Orthodox churches hold and much of the Reformation rejected or reinterpreted, making the transmission of apostolic office one of the central disputed questions of Christian order. The word has also been used more loosely, for the first missionaries who brought the faith to a nation, called the apostle of that people, and some modern movements have revived the title for present leaders. The tension between the unrepeatable original office and the claim to continue it runs through the whole history of the church.