Spiritual Gifts Part 271: Hold fast what you have, so that no one will take your crown.

3. Reward:
to each according to his deeds; crowns.
MAT 5:11-12; MAT 10:40-42; LUK 6:35-36; 1CO 3:8-15; COL 3:23-24; JAM 1:12; 2JO 1:8; REV 2:10; REV 22:10-14

At the Bema Seat judgment, the overcoming believers will be promoted.

pleres :
a totality which has been brought to completion - 'whole, complete, full.' abounding in.
lacking nothing, perfect

JOH 1:14
And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth.

In context here with the Philadelphia church, “what you have” is the truth in your souls, the word of God.
“you have kept My word”

“hold fast” =
krateo
to use strength, i.e. seize or retain (literally or figuratively)

krateo =
to be able to complete or finish, presumably despite difficulties - 'to accomplish, to do successfully, to carry out.'

The difficulties are related to the angelic conflict, and our enemies in that struggle: the world, the flesh, and the devil.