I do not doubt there is a consecutive history in the passage - which I divide into two parts, chapters 2 and 3 - for it ends in Thyatira or popery. In chapter 3 we have Protestantism alongside. Philadelphia is a church without pretension which keeps Christ's word and does not deny His name; which further keeps the word of His patience; which still expects Christ, though it may seem He will never come; He is waiting, and in the patience in which He waits for the long-suffering of God is salvation, He is not slack concerning His promise - taking His word to guide and still waiting; and such will be kept. But it is not the party which outwardly characterises the church which is addressed - all, in a general sense - but (I believe) those who have ears to hear.
It is not apostasy which characterises Laodicea, nor heresy, nor Babylon, but much worse, I think; professed light from human sources, from the human mind - and has not God's eye-salve (nor gold tried in the fire, Christ as divine righteousness), the sense of the value of all things. It is just the reality of that which is divine, known by divine teaching.
I do not think the seven churches go down a regular declivity. Smyrna is God's stopping declining by persecution. Philadelphia is not decline on Sardis. The tendency is there, but it is not absolute, or universal. A great deal that is neither here nor there has been said as to Philadelphia and Laodicea; but those of Philadelphia are not the description of the progress of evil. Not keeping Christ's word, but denying His name, was their danger; and in this they had overcome: the other was dropping the expectation of the Lord, the word of His patience; in this, too, they had overcome: and they had two promises - kept from the hour of temptation - they would be off before it (not διά; ἐκ) and the ecclesiastical powers which had despised them should be humbled to recognise that Christ had loved them. In verse 12 they are singularly identified with Christ. But the faithful in Philadelphia are called to overcome as much as in Laodicea. Faithfulness in the circumstances of each particular assembly is what each are respectively called to.