The gospel according to the wiki god. Go figure....
Yes, I am aware of the original twelve and that many things did transpired after the Savior left. Even Paul, there is no direct ordination of him being called to Apostleship. In fact, Luke was called many times as a physician by Paul. Seeing the two largest writings were completed by Luke [Acts], I have no doubt being a earlier scribe, he was in no doubt called. Some non-canonize work has indicated it since he was mentioned frequently.
On the level of likelihood, a number of careful scholars ask about the authorship of Luke:
"If people were guessing, would they not be much more likely to come up with an apostle?" And on the restricted question of fact, Luke is the only author mentioned by the prominent church fathers and important hand-written copies of the Gospel in the early Christian centuries.
Paul profiles Luke. Besides giving Luke's general greetings in two letters (Col. 4:14; 2 Tim. 4:11), the apostle was specific at the end of Colossians, describing him as "the beloved physician" (Col. 4:14). While Christian leaders regularly addressed their converts by "beloved," this term of endearment applied to a fellow laborer amounts to a designation of intense trust. Paul was most sensitive about who instructed the volatile branches, so here he really designated Luke as an apostolic associate whose spiritual—and historical—knowledge could be trusted. This relationship gives important color to Luke's preface; he could record what apostles knew because he was their intimate companion.