Vincent's NT Word Studies
15. These are they which (outoi eisin oitinev). Which denotes them as belonging to a class. Hence Rev., rightly, such as.Honest and good heart. Peculiar to Luke. Honest; lit., fair, noble.
Honest, not in the popular sense, but in the sense of the Latin honestus; noble, virtuous, worthy.
Keep (katecousin). Much better Rev., hold it fast, giving the force of the compound verb.
With patience. Or in patience. Peculiar to Luke. In contrast with fall away, ver. 13.
Robertson's NT Word Studies
8:15 {In an honest and good heart} (en kardiai kalei kai agaqei). Peculiar to Luke. In verse #8 the land (gen) is called agaqen (really good, generous) and in verse #15 we have en tei kalei gei ({in the beautiful or noble land}). So Luke uses both adjectives of the heart. The Greeks used kalos k' agaqos of the high-minded gentleman. It is probable that Luke knew this idiom. It occurs here alone in the N.T. It is not easy to translate. We have such phrases as "good and true,"sound and good,"right and good," no one of which quite suits the Greek. Certainly Luke adds new moral qualities not in the Hellenic phrase. The English word "honest" here is like the Latin _honestus_ (fair, noble). The words are to be connected with "hold fast" (katecousin), "hold it down" so that the devil does not snatch it away, having depth of soil so that it does not shrivel up under the sun, and is not choked by weeds and thorns. It bears fruit (karpoforousin, an old expressive verb, karpos and forew). That is the proof of spiritual life. {In patience} (en hupomonei). There is no other way for real fruit to come. Mushrooms spring up overnight, but they are usually poisonous. The best fruits require time, cultivation, patience.