This can be done within 2 generations pretty reliably. Easiest way is to become the Byzantine Emperor and then get the Kingdom Titles for Sicily and Hellas; convert the 7 counties in Hellas to Christianity and you will get the achievement. Otherwise you are going to have a long fight to build up strong enough to take over Sicily and one of the Greek Kingdoms either Hellas (7 counties), Eprius (11) or Thessalonika (17) and then convert all of the counties in one area to Christianity.
When starting as Robert the Fox start down the stewardship lifestyle and put one point in Meritocracy and from that point on go down the intrigue tree to get Kidnapper.
Swear fealty to the Byzantine Empire which will keep you safe and also mean you can use the claim throne scheme. Once that is done and you can start a kidnap scheme, kidnap the current Emperor. As of patch 1.1.2 the max chance is 80% but it doesn't take too long to restart if it fails. Pause the game when you are bribing agents and get all of them on side, the debt shouldnt be too bad.
Once you kidnap the Emperor start a faction to to claim the throne and press your demands. This will start a war and because you have the Emperor in your cell you will get 100% straight away. As the Emperor you can use a decision to reclaim Constantinople, do this as it will make earning money easier.
You may need to convert to the Orthodox religion and Greek culture to prevent uprisings which is fine. Save up enough money to buy the Kingdom of Sicily and Hellas. You may now be playing your son who you should make Orthodox and start making all your vassals happy. Once everyone is happy get 250 piety and convert to Christianity. Most of you vassals will convert as well depending on how much they like you and potentially all of Hellas will be Christian in one click.
If not put perks down the Theology tree to speed up conversion and turn the last few counties to Christianity. You don't need to own the counties in Hellas just the Kingdom title.