How do you know that the Bible is the Word of God?

"How do you know the Bible is the Word of God?" 
This was asked by a Muslim.
Muslims sometimes think the Bible has the same status in Christianity as the Quran does in Islam.
Really, if you think about it, there were Christians before there was any Bible.
The central point about Christianity is that Jesus rose from the dead.
Paul tells us that when he came to the Corinthians, he came with a short creed about Jesus that he received from Peter when he visited Jerusalem.

For I delivered to you as of first importance what I also received,

• that Christ died for our sins according to the Scriptures,  
• and that He was buried, and that He was raised on the third day according to the Scriptures,
• and that He appeared to Cephas, then to the twelve.
• After that He appeared to more than five hundred brethren at one time,

most of whom remain until now, but some have fallen asleep;  

• then He appeared to James, then to all the apostles; and

last of all, as to one untimely born, He appeared to me also. For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God.  But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me.  Whether then it was I or they, so we preach and so you believed.
(1 Cor 15)

Why else would Paul mention that most of the five hundred are still alive unless as a challenge to the Corinthians to investigate his claims if they did not believe him?

Therefore Christianity was verifiable to the earliest Christians, and that is why they believed--more so than from the Bible being the Word of God.

All this is really better explained in this video.

William Lane Craig on Evidence for the Resurrection