I know many will shoot me, and I have since years this in my mind, but I can't name him the GOAT as many people tend to.
Of course tiki-taka Barcelona was something spacestellar, but he was more a manager than a builder coach. I mean you don't need to be the master tactician to play prime Xavi, prime Iniesta, prime Villa, prime Alexis Sanchez, Prime Messi, etc, etc.
For me where he failed to seal his "godly" status, was his next team, although it was also an Overpowered team, Bayern München.
He received a fine tuned Bayern from Jupp Heynckes, playing a panzer style gegenpress physical foootball, accompanied by BvB, a style that was based more on, Teamgeist, stamina (
despite people saying it was dependent on medicines) and less on technique, which lead in a dominant German football for 2 years, 2013 CL final BvB-FC Bayern, and 2014 WC conquest by Germany.
What Guardiola did to this? He shuttered completely the Bayern panzer mentality, forcing them to play tiki-taka, which never stack to Bayern, so he ruined the previous work.
Of course he dominatesd in Bundesliga, but in Champions League he failed hard, and was heavily criticized for his tactics, tiki-taka Bayern.
Then again in Man City, he won some more titles, but not the Champions League. Though he changed his tiki-taka only mentality, to something more mix and match with British high pressure and more direct attacking tendencies.
For me to consider him a Messiah, I would expect him to do what
@mattmid said, or to take a lesser tier team, and while applying his mentality, do what Mourinho did with FC Porto.