Can a disciple of Jesus have a relationship with someone who is not?

Jesus certainly had relationships with people who were not his
followers. How can we influence the world if we are not willing to have
relationships with those who are in the world.

I am guessing that you are asking if it is acceptable for a
faithful follower of Jesus to form a romantic relationship with someone
who is not a believer in Jesus. The plain answer is no. There is no
single passage which absolutely and unambiguously says that it is a sin to
have a relationship with a non-Christian, but there are many biblical
principles which makes it very clear. Let me mention just a couple.
First, one might mention 2 Corinthians 6:14. “Do not be yoked together
with unbelievers.” In the context, Paul is not speaking specifically
about marriage, but it is hard to think of a more appropriate application
of this passage than to romantic relationships of a disciple of Jesus with
a non-believer. In 1 Corinthians 15:33 Paul tells us, “Do not be misled,
bad company corrupts good character.” Again, his is not referring
specifically to marriage, but it is hard to think of a more likely chance
for a non-believer to corrupt one’s faith in Jesus than for us to marry
one!

One might say, “Well, I am not going to marry him (or her), I just
need some friendship in my life. Besides, I am trying to help him (or
her) to become a Christian.” These are the words of a fool. There is no
one more foolish than a man or woman in love. I would caution anyone who
is a Christian and who is even considering forming a romantic relationship
with a non-believer that they are playing with fire. Those who play with
fire will get burned.

John Oakes