Therefore, they, being united to that, do not decay; nor do they fall from that. For what is Brahman, that alone grows. That which is Brahman, that alone grows.

Sannihita — united, close, established in. One who is united to Brahman does not decay (na apahīyate), does not fall (na upahīyante).

The word vardhate — “grows, flourishes, prospers” — is key. Brahman is not static; it is dynamic, expansive. It grows, and one who is united to it grows with it.

The repetition “yat hi brahma eva vardhate, tat hi brahma eva vardhate” — “for what is Brahman, that alone grows” — indicates that Brahman is the only reality that truly grows. Everything else has cycles of growth and decay; only Brahman grows eternally.

For the yogī: spiritual growth is not accumulative (more knowledge, more powers), but identificatory — to grow in Brahman is to let Brahman manifest more fully through oneself.