What is the best way to introduce a Christian to Islam?

     When the Prophet, s.a.w., sent Mu'adh ibn Jabal, r.a., to
Yemen, he said to him: "You are going to a people from the People of
the Book. So let the first thing that you call them to be the
testification ( that there is no God but ALLAH and that Muhammad,
s.a.w., is His Messenger )- if they accept that from you, then
inform that ALLAH has made it obligatory upon them 5 daily prayers
in a day and a night. If they accept that from you, then inform that
ALLAH has obligated on them a charity ( Zakat ) that is taken from
the rich and given to the poor."

[ related by `Abdullah ibn Abbas, r.a., in Sahih Bukhari 5:109 ]
When Ali, r.a., was given the flag on the day of Khaibar, the
Prophet, s.a.w., said: "Be patient and invite them to Islam. Let
them know what is obligatory on them of ALLAH's rights, for I swear
by ALLAH, if one man was to be guided through you, it would be
better for you than the red camel." [ related by Sahl ibn Sa'd,
r.a., in Sahih Bukhari 5:77 ] Remember, in that time, the red camel
was equivalent to a Lexus today!

The greatest sin in the Bible:

     So the first thing one should acquaint a Christian with is the
concept of Tawheed ( the belief that, not only is ALLAH One but how
He is One ) and how one cancels their belief in Tawheed ( which is
called Shirk ). This is the greatest sin one can commit, even
according to the Bible, for, on different occasions, Jesus told both
the scribe and the lawyer who asked him "What is the greatest
commandment in the law?": "Hear, O Israel; The Lord our God is One
Lord . . . and the second is like, namely this: Thou shalt love thy
neighbor as thyself. There is none other commandment greater than
these." ( Mark 12:28-31. ) Further, he refers to "The Lord our God
is One" in Matthew 22 as "the greatest commandment" and he says: "On
these two commandments hang all the law and the ( teachings of the )
prophets." Well, if that is the greatest commandment, surely to
break it would be the greatest sin anyone can commit!

     ALLAH says in the Qur'an: "Whosoever sets up partners in
worship with ALLAH, then ALLAH has forbidden Paradise for him and
the Fire will be his abode." ( -5:72 ) "If anyone assigns partners
to ALLAH, it is as if he had fallen from the sky, and the birds had
snatched him, or the wind had throw him to a far-off distant place."
Qur'an-22:31

     "Say: `O People of the Book ( i.e., Jews and Christians )! Come
to common terms as between us and you: That we worship none but
ALLAH (alone); that we associate no partners with Him; and that none
of us shall take others as Lords besides ALLAH.'  Then if they turn
away say: `Bear witness that we are Muslims.'" -Qur'an, 3:64.

      But this is in their Book as well. Isaiah 45:5 states: "I am
the Lord, and there is none else, there is no God beside Me." I
Timothy 6:15-16 says: "The King of Kings and Lord of Lords; who only
hath immortality, dwelling in the light which no man can approach
unto; whom no man hath seen, nor can see: to whom be Honor and Power
everlasting." [ See also St. John 5:37 and Qur'an 6:103  where it
says "no vision can grasp Him but His grasp is over all
vision."  "He neither slumbers nor sleeps." ( Qur'an 2: 255 & Psalms
121:2-4 ) For "GOD is NOT a MAN, that He should lie; nor the SON OF
MAN ( Jesus ) that He should repent." ( Numbers
23:19 ) . . . . .  "Before Him, there was no God formed, nor shall
there be after Him." ( Isaiah 43:10 ) "You have neither heard His
voice at any time nor seen his shape." ( St. John 5:37 ) "But will
God indeed dwell on the Earth? Behold the Heaven and the Heaven of
Heavens cannot contain thee; how much less this house that I have
built?"  ( I Kings 8:27, Acts 17:24-25 )