OF THE FOUR EVANGELISTS, Luke wrote the best Greek and, unlike the other three, was almost certainly a Greek-speaking Gentile himself, who put his Gospel together for a Gentile audience, translating Jewish names and explaining Jewish customs when he thought they wouldn't be understood if he didn't. In his Letter to the Colossians, Paul refers to somebody as "Luke the beloved physician," and without stretching things too far, you could point to three blocks of material in Luke's Gospel, omitted from the others, that might suggest that he was the same man.
First of all, there's the parable of the Prodigal Son, the account of the whore who washed Jesus' feet and dried them with her hair, and the scrap of conversation Jesus had with one of the two crooks who was crucified with him.
Smelling of pig and cheap gin, the Prodigal comes home bleary-eyed and dead broke, but his father's so glad to see him anyway that he almost falls on his face. Jesus tells Simon the blue-nosed Pharisee that the whore's sins are forgiven her because, even painted up like a cigar-store Indian and smelling like the perfume counter at the five-and-dime, she's got more in her of what the gospel of love is all about than the whole Ladies' Missionary Society laid end to end. The thief Jesus talked to on the cross may have been a purse snatcher and second-story man from way back, but when he asked Jesus to remember him when he made it to where he was going, Jesus told him he'd make sure they got rooms on the same floor. Different as they all are in some ways, it's not hard to see that they all make the same general point, which is that, though he could give them hell when he felt like it, Jesus had such a soft spot in his heart for the scum of the earth that you would have almost thought he considered them the salt of the earth the way he sometimes treated them.
Second, Luke is the one who goes out of his way to make it clear how big Jesus was on praying. He prayed when he was baptized and after he healed the leper and the night before he called the twelve disciples, and Luke was the only one to mention these together with a few others like them and also was the only one to say that the last words Jesus ever spoke were the prayer, "Father, into thy hand I commend my spirit." It's also thanks to Luke that there's a record of the jokes Jesus told about the man who kept knocking at his friend's door till he finally got out of bed to open it and the widow who kept bugging the crooked judge till he finally heard her case just to get a little peace, the point of both of which seems to be that if you don't think God has heard you the first time, don't give up till you're hoarse. Luke wanted that to be remembered too.
Third and last, Luke makes sure that nobody misses the point that Jesus was always stewing about the terrible needs of poor people. He is the one who tells us that when Jesus preached at Nazareth, his text was "he has appointed me to preach good news to the poor" from Isaiah (Luke 4:18), and whereas Matthew says that the first Beatitude was "Blessed are the poor in spirit," according to Luke it was just plain "Blessed are the poor" period (Luke 6:20). He also recorded some parables, like the one about the rich man and the beggar, that come right out and say that if the haves don't do their share to help the have-nots, they better watch out, and he's the only one to quote the song Mary sang that includes the words "he has filled the hungry with good things, and the rich has sent empty away" (Luke 1:53).
To put it in a nutshell, by playing all these things up Luke shows he was a man who believed that you shouldn't let the fact that a person is jailbait keep you from treating that person like a human being, and that if you pray hard enough, there's no telling what may happen, and that if you think you've got heaven made but don't let it worry you that there are children across the tracks who are half starving to death, then you're kidding yourself. These characteristics may not prove that he was a doctor, like the Luke in Paul's Letter, but if he wasn't, it was a serious loss to the medical profession.