Praying behidn the Murji'ah is not allowed! It was narrated from Imam Muhammad bin Aslam that he said: “I heard Yazid ibn Harun say, “Whosoever is a caller/scholar of irjaa’, the prayer behind him has to be repeated.” [Al-Laalika’i 5/1067] Does this mean that that the Murjiah are disbelievers! May Allaah reward you Shaykh.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

The Murji’ah are divided into sub-sects as it is known in the Books of Sects. Some Murji’ah jurists consider that actions are not included in the circle of faith, but they consider sinful the one who does not carry out an obligation or commits what is forbidden. However, the difference of opinion with them (Murji’ah jurists) is a verbal dispute, as stated by Shaykh Ibn Taymiyyah .

Also, some of them consider that sins do not undermine the deeds of a person as long as he is a believer, in the same manner that a good deed does not benefit a disbeliever.

Also, there are among them extremists who believe that faith is just knowledge (belief in Allaah), and they are not disbelievers but innovators. However, they are not all equal in innovation [i.e. their degree of innovation varies]. For more benefit, please refer to fataawa 91651 and 10346.

In any case, the prayer behind them is valid because the most preponderant opinion is that the prayer is valid behind the people of the Qiblah regardless of whether they are righteous or dissolute.

Perhaps the scholars who forbid the prayer behind them held the view that the prayer behind a dissolute person is void.

Therefore, the quote that you have mentioned in the question does not imply that the one who said this quote considers the Murji'ah to be disbelievers. The preponderant view is that of the majority of the scholars; that the prayer is valid behind any righteous or dissolute Muslim.

Allaah knows best.