en_tn_lite_do_not_use/jas/01/04.md

Let endurance complete its work

Here endurance is spoken of as if it were a person at work. Alternate translation: "Learn to endure any hardship" (See: rc://en/ta/man/jit/figs-personification)

fully developed

able to trust in Christ and obey him in all circumstances

not lacking anything

This can be stated in the positive. Alternate translation: "having all that you need" or "being all that you need to be"