If I shake hand with the Christian man while I going to pray, I have to repeat the Wudu again as they are Najis, because they didn't clean themselves after finishing evacuation from the toilet, and they never wash their hands. Please I waiting for your kind answer.

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. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If a Muslim shakes the hands of a non-Muslim, whether a Christian or anyone else, his ablution is not invalidated. The impurity of the non-Muslim is abstract, because his soul is impure as a result of his Kufr (disbelieving in Allaah). This is the meaning of the verse of Allaah (which translates): {Verily, the disbelievers (are impure}[9:28]. So it is not a corporeal impurity. Moreover, some scholars stated that if one is pretty sure that the clothes of the non-Muslims are touched by an impurity; they should not be worn unless they are washed. So if one shakes the hand of a non-Muslim and his hand gets wet by what he thinks is an impurity, then it is more appropriate to wash his hand. But his ablution is not void because of touching the impurity, but he has to get rid of it [by washing what is affected by it].

Allaah knows best.