Mustard - Vine's Expository Dictionary of New Testament Words
Mustard[ 1,,G4615, sinapi ]
a word of Egyptian origin, is translated mustard seed" in the NT. "The conditions to be fulfilled by the mustard are that it should be a familiar plant, with a very small seed, Matthew 17:20; Luke 17:6, sown in the earth, growing larger than garden herbs, Matthew 13:31, having large branches, Mark 4:31, ... attractive to birds, Luke 13:19 [RV, '(became) a tree']. The cultivated mustard is sinapis nigra. The seed is well known for its minuteness. The mustards are annuals, reproduced with extraordinary rapidity ... In fat soil they often attain a height of 10 or 12 feet, and have branches which attract passing birds" (A. E. Post, in Hastings' Bib. Dic.)
The correct RV translation in Matthew 13:32, "greater than the herbs," for the AV, "greatest among herbs" (the "mustard" is not a herb), should be noted.
As the parable indicates, Christendom presents a sort of Christianity that has become conformed to the principles and ways of the world, and the world has favored this debased Christianity. Contrast the testimony of the NT, e.g., in John 17:14; Galatians 6:14; 1 Peter 2:11; 1 John 3:1.