A modernist wants to know how could Hadhrat Umar (radhiallahu anhu) have prohibited mut'ah which was allowed by Rasulullah (sallallahu alayhi wasallam

Answer

Ask the modernist from whence did he get the idea that it was Hadhrat Umar (radhiyallahu anhu) who had first prohibited it? Even if it was Hadhrat Umar (radhiyallahu anhu), then too, the entire Ummah, including the Sahaabah, submitted and accepted the validity of his decrees. But the factual position is that it was Rasulullah (sallallahu alayhi wasallam) who had unequivocally prohibited mut'ah. The modernist's problem is that despite lacking Islamic knowledge, he reads shiah literature which appeals to the nafs, hence he joined the Shiahs in the slander against Hadhrat Umar (radhiyallahu anhu).