Jamieson, Fausset and Brown on Revelation 3:8

8. I have set--Greek, "given": it is My gracious gift to thee.

open door--for evangelization; a door of spiritual usefulness. The opening of a door by Him to the Philadelphian Church accords with the previous assignation to Him of "the key of David."

and--The three oldest manuscripts, A, B, C, and Origen read, "which no man can shut."

for--"because."

a little--This gives the idea that Christ says, He sets before Philadelphia an open door because she has some little strength; whereas the sense rather is, He does so because she has "but little strength": being consciously weak herself, she is the fitter object for God's power to rest on [so Aquinas], that so the Lord Christ may have all the glory.

and hast kept--and so, the littleness of thy strength becoming the source of Almighty power to thee, as leading thee to rest wholly on My great power, thou hast kept My word. Grotius makes "little strength" to mean that she had a Church small in numbers and external resources: "a little flock poor in worldly goods, and of small account in the eyes of men" [Trench]. So Alford. I prefer the view given above. The Greek verbs are in the aorist tense: "Thou didst keep ... didst not deny My name": alluding to some particular occasion when her faithfulness was put to the test.