John Chrysostom

"Luke, the beloved physician, saluteth you." This is the Evangelist. It is not to lower this man that he placeth him after, but to raise the other, viz. Epaphroditus. It is probable that there were others called by this name. "And Demas," he says. After saying, "Luke, the physician, saluteth you," he added, "the beloved." And no small praise is this, but be beloved of Paul even great exceedingly.

Homily on Colossians 12 ↗

Theophylact of Ohrid

Luke is the evangelist, for whom it is no small praise to be beloved by Paul. He mentions him after Epaphras — not because he wished to demean him, but because he desired to exalt Epaphras before his fellow citizens. Undoubtedly, there were also others who bore this name. Demas, as can be seen, had not yet abandoned the teacher.

Commentary on Colossians ↗

Irenaeus

That he was not merely a follower, but also a fellow-labourer of the apostles, but especially of Paul, Paul has himself declared also in the Epistles, saying: "Demas hath forsaken me, ... and is departed unto Thessalonica; Crescens to Galatia, Titus to Dalmatia. Only Luke is with me." From this he shows that he was always attached to and inseparable from him. …

Against Heresies Book III ↗

Thomas Aquinas

Then he mentions Luke, who it seems, was not Jewish by birth, because he was from Antioch, and a physician. He singles him out because he had great authority in the Church because of the Gospel he wrote while the Apostle was still alive. He also mentions Demas.

Commentary on Colossians ↗