^ The thing is, look at how it all fell into place and it kinda makes sense...
Cersi got Margeri locked up meaning Cersi had power for a little while, then after Cersi went to visit Margeri to throw dirt in her face, she gets summoned by the High Sparrow to talk, and later thrown in jail herself...
Now there is no one who can talk to the king directly that can influence him, and since hes in such a sad sorry state for himself right now (locking himself in his room unwilling to talk to anyone), there is basically no one to rule and give orders, leaving the High Sparrow free to run wild as he pleases...
Add to that the Queen Mother Cersi was the one that gave the high sparrow his power, and even the guards in the city were turning a blind eye to what they have been doing, and as has been seen in the series, it seems they (the sparrows) are gaining more and more followers who are common people...
So it all adds up that his power could have built up that quickly, especially from inside Kings Landing when he had the authority from the Queen Mother herself...
Last but not least, lets not forget, Cersi knew when she promoted the High Sparrow that he said, no man, common or king shall rise above gods law... so basically they are all screwed regardless what or who you are...
Cersi got Margeri locked up meaning Cersi had power for a little while, then after Cersi went to visit Margeri to throw dirt in her face, she gets summoned by the High Sparrow to talk, and later thrown in jail herself...
Now there is no one who can talk to the king directly that can influence him, and since hes in such a sad sorry state for himself right now (locking himself in his room unwilling to talk to anyone), there is basically no one to rule and give orders, leaving the High Sparrow free to run wild as he pleases...
Add to that the Queen Mother Cersi was the one that gave the high sparrow his power, and even the guards in the city were turning a blind eye to what they have been doing, and as has been seen in the series, it seems they (the sparrows) are gaining more and more followers who are common people...
So it all adds up that his power could have built up that quickly, especially from inside Kings Landing when he had the authority from the Queen Mother herself...
Last but not least, lets not forget, Cersi knew when she promoted the High Sparrow that he said, no man, common or king shall rise above gods law... so basically they are all screwed regardless what or who you are...