Does this parable mean that seeds of the Kingdom are to be sown indiscriminately by Christians?
Not indiscriminately. Widely spread. Yes making God’s word known widely to all that would be receptive. The Holy Spirit will work in God’s own in due time. I will go a bit deeper. Here is what Jesus said; Mark16: 15 And he said unto them, Go ye into all the world, and preach the gospel to every creature. 16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned. This makes it the choice of the creature. What is then chosen? Ephesians 1: 4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love: 5 Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will, Many Christians were chosen before the foundation of the world – these battled against the adversary. God Almighty knows who these are. Paul was one of these and stated he was a slave/servant of God. Understand this about Saul. He was a young man, a rich yo