711. That 'entering into the ark' means being made ready has been stated already at verse 18 of the previous chapter. There however it meant being made ready for salvation as regards things of the understanding, which are the truths of faith, whereas here it means being made ready as regards those of the will, which are goods springing from charity. Unless a person has been made ready, that is, has been supplied with truths and goods, he cannot possibly be regenerated, still less undergo temptations, for the evil spirits who are at that time with him activate the falsities and evils that are his. And unless truths and goods are present towards which they are turned by the Lord and by which they are scattered, he goes under. The truths and goods are the remnants which the Lord preserves for such purposes.

'Grass' is natural truth, and 'flower of the field' is spiritual truth, as in Isaiah 40:5, 6. Green grass, in the Word, signifies the good and truth of the church, and of faith, which first springs up in the natural man. This is also signified by 'herb in the field.' Grass and the pulse of the herb, as in Psalm 32:2, signify what is most vile. Grass signifies science from a spiritual origin, or that by which spiritual truth is confirmed. 'Reeds and rushes' signify science from a sensory origin, or the things which confirm the fallacies of the senses, as in Isaiah 25:7. This science, considered in itself, is only a part of the lowest natural degree, which may be justly called material and bodily, in which there is little or no life. Green grass also signifies that which is alive with man. 'Grass burnt up' signifies the things which are dead in him. Grass signifies the scientific principle. Just as green grass serves animals for support, spiritual truth serves men for spiritual nourishment.

(References: Apocalypse Explained 507)