Theophylact of Ohrid
Catena Aurea · Markusevangelium 4
And he said, Whereunto shall we liken the kingdom of God? or with what comparison shall we compare it? It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth: But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it. And with many such parables spake he the word unto them, as they were able to hear it. But without a parable spake he not unto them: and when they were alone, he expounded all things to his disciples.
For since the multitude was unlearned, he instructs them from objects of food and familiar names, and for this reason he adds, But without a parable spake he not unto them, that is, in order that they might be induced to approach and to ask Him. It goes on; And when they were alone, he expounded all things to his disciples, that is, all things about which they were ignorant and asked Him, not simply all, whether obscure or not.