The question is was the work of daawah part of Prophet Easa’s a.s. syariat? Or is the current Christian missionary work of the Christians an invented phenomenon?

Every single prophet was sent into this world for the guidance of man. Therefore, the work of Da'awah was the actual object behind the deputation of prophets onto the earth. However, unlike our Ummah, the people of the previous nations never did the work of Da'wah as widely as possible.

Nevertheless, even if missionary work took place previously as it takes place presently, this will in no way whatsoever, give them superiority or make them equal to our Ummah for the simple reason that Christianity has been abrogated by Islam. Our Ummah being the best of all nations is based on its responsibility - the work of Da'awah - although the Christians are aldo doing missionary work. It is of no use at all, neither will they be rewarded for it - in fact they will be taken to task, simply because Christianity has been abrogated by Islam.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai