Calling people to Islam
means establishing personal relationships with the Kuffaar, in the first
place to break the ice and pave the way for da’wah. If one of them
invites me to eat or drink something that contains nothing haraam, such as
cheese, fish and tea, is it permissible for me to eat or drink it? What if
there is the possibility that the vessels have been used before that to
serve pork or wine – even though they have been washed with soap and
water?

Praise be to Allaah. 

Relationships
between people are of different kinds. If the Muslim has a relationship of
friendship, love and brotherhood with a kaafir, this is haraam and may be
kufr. Allaah says (interpretation of the meaning):

“You
(O Muhammad) will not find any people who believe in Allaah and the Last
Day, making friendship with those who oppose Allaah and His Messenger
(Muhammad), even though they were their fathers or their sons or their
brothers or their kindred (people). For such He has written Faith in their
hearts, and strengthened them with Rooh (proofs, light and true guidance)
from Himself. And He will admit them to Gardens (Paradise) under which
rivers flow, to dwell therein (forever). Allaah is pleased with them, and
they with Him. They are the party of Allaah. Verily, it is the party of
Allaah that will be the successful” [al-Mujaadilah 58:22]

And there are
other aayaat and ahaadeeth which convey the same meaning.

 If
it is a relationship based on selling or buying, or accepting an
invitation to eat halaal food, or accepting a gift of something that is
permissible, for example, without that having an influence on the Muslim,
then this is permitted and it is permissible to eat the food offered by a
kaafir to a Muslim if the food and drink are halaal. Even if it is
presented in a vessel which has previously been used for drinking wine or
eating pork, etc., it is still permissible to take it, so 
long as the vessel has been washed after being used for haraam or
naajis (impure) things and all traces of them have been completely
removed. If this will help to convey the message of Islam, this is a
stronger reason why these invitations should be accepted and these
relationships upheld, and there is more hope that there will be a reward
in this case.