How do you know that Jesus walked on water?
Atheist philosopher Peter Boghossian is fond of asking Christians that and other similar questions, such as how do you know that Jonah was swallowed by a large fish?  When met with the reply, "Because the Bible says so," Boghossian reminds his Christian audience that Muslims believe Muhammad flew to Heaven on a winged horse because of the "night journey" chapter contained within the Koran.
Back in late 2013, Boghossian released his book entitled, A Manual for Creating Atheists
where he redefines faith to be "pretending to know things that you don't know" and "belief without evidence."  He calls faith "an unreliable epistemology,"  a "virus," and calls for a process and agenda that will "ultimately eradicate faith." 
Let's take a brief look at Boghossian's claims to see if he's right about what faith really is and see how Christians should answer him on Jesus walking on water and other similar questions.
A Flawed Foundation
It's strange that someone who champions evidence and empirical inquiry for validating truth claims chooses to invent out of thin air the foundation on which he builds his case. Boghossian's definition of faith is in reality a twisted redefinition of the term, and his assertion of faith being an epistemology (a system or study about how one acquires knowledge) is simply incorrect.
Taking the last error first, faith is not an epistemology or a way of acquiring knowledge. Most introduction to philosophy textbooks will tell you there are at least five ways  of gaining knowledge that justify a particular belief or position that a person might hold, none of which equate to Boghossian's characterization of faith:
1. Rationalism — through reason alone.
2. Subjectivism — through human intuition and contact with the subject itself.
3. Testimony/authoritarianism — through witnesses and authorities on the subject.
4. Empiricism — through the natural sciences alone.
5. Existentialism — through experience alone.
In no way is faith an epistemology, but rather it is a trust based on prior information about someone or something, which has been gained through the use of one or more valid ways of acquiring knowledge. Boghossian fails in not recognizing (either willingly or unwillingly) this fact and builds right from the start on a flawed foundation.
A Flawed Definition
This leads us to Boghossian's disingenuous redefinition of the term "faith," which he says is "pretending to know things that you don't know" and "belief without evidence." His first redefinition is concocted out of his own imagination and is illogical,  with his second subtly altering one of the definitions that is found in most dictionaries.
Look up "faith" in any dictionary and you will be presented most likely with the following: 
1. confidence or trust in a person or thing: faith in another's ability.
2. belief that is not based on proof.
The first definition always supersedes the second (and for good reason), but it is not put forward by Boghossian as a definition of faith because it does not suit his purpose.
He changes the second definition to be "belief without evidence
" vs. "belief that is not based on proof
" in hopes of dissolving the important differences between "proof" and "evidence" and advancing the false notion that worldviews like Christianity contains no valid evidential foundation. Any law enforcement expert will tell you there is a big difference between "proof" and "evidence" and that it is through a cumulative case method of gathering multiple pieces of evidence that a person arrives at a conclusion on whether a truth claim is correct or not. 
Boghossian also errors by performing no Biblical morphological word study of "faith." If one is going to tell billions of people what their term for "faith" means, you would think he'd work through that exercise vs. simply presenting one or two verses from the Bible and leaving it at that.
But there's a reason he doesn't.
In the Greek language there are a number of words that could have been used to reference faith in the New Testament. The Hellenistic and classical Greeks used the term nomizo
to describe faith in their gods. The word basically means "I believe" only because something was passed along by tradition (e.g. by parents, etc.). In other words, there is no real rationale for the belief — a definition that dovetails quite nicely with how Boghossian and other atheists want to define "faith" in Christianity.
Unfortunately for them, that word is never used in the Greek New Testament to speak of faith.
Instead, the term pistis
is used in Scripture. It is a noun that comes from the verb peitho
, which means "to be persuaded." If you check the best lexicon (BDAG) for the meaning of pistis
, you'll find the following:
• State of believing on the basis of the reliability of the one trusted
• Trust, confidence, that which evokes trust
• Reliability, fidelity, pertaining to being worthy of belief or trust
The same is true of the Hebrew term for faith (ěměṯ
), which denotes firmness, trustworthiness, constancy, duration and truth. 
Boghossian's redefined faith of "belief without evidence" is foreign to both the Old and New Testaments and unsuccessfully impacts Christianity because (1) it fails to understand that faith is a trust given in response to acquired knowledge as we'll see in just a minute, and (2) it fails to make the important distinction between faith in something and faith that something exists
A Flawed Understanding
Now let's return to the question of how to know if Jesus walked on water. The process used to answer this question is no different than the one needed for tackling any other question from history. How do you know that anything from history actually happened?
Look back at the five epistemological methods for acquiring knowledge and ask which one(s) are appropriate to use for tackling this question. Although a mixture of more than one is possible, the method of testimony is really the one that historians foundationally rely on to ascertain the truthfulness of a non-repeatable event/historical report. You have to rely on eyewitness accounts that were accurately recorded by trustworthy individuals.
But how do you know if the testimony is accurate and/or if the witnesses are telling the truth? The Scottish skeptic David Hume (no believer in God) proposed the following tests: "We entertain a suspicion concerning any matter of fact when the witnesses contradict each other, when there are few of them or they are of a doubtful character, when they have something to gain by their testimony, when they deliver their testimony with hesitation or with over-violent conﬁdence." 
Let's apply Hume's criteria to Jesus walking on water:
• Is there contradiction among the witnesses? No, the multiple attestations agree on the key facts that Jesus walked on the sea.
• Are there only a few witnesses? No, there were a dozen who witnessed the event.
• Do the witnesses have doubtful character? No, they were committed to a religion that espoused high standards of truth and characterr.
• Did they have something to gain from their testimony? No, if anything they had much to lose, which history says they did.
• Is the testimony hesitant or overzealous? No, the multiple accounts display no hesitancy or extra zeal; the accounts simply state the facts.
Let me pause just for a moment and quickly comment on the real problem Boghossian and other atheists have with Jesus walking on the sea — it's a miracle
. It has nothing to do with epistemologies, etc., but everything to do with a worldview that starts with the premise that God does not exist and miracles don't happen. That is an entirely different matter to debate.
However, let's re-engage now with the matter at hand and conclude with Boghossian's supposed proof text that shows how Biblical faith fits his redefinition of the term: "Now faith is the substance of things hoped for, the evidence of things not seen" (Hebrews 11:1). Doesn't this support his case?
Not at all. Instead, the Bible plainly shows us in many places what the writer of Hebrews means when he says faith is the "evidence of things not seen." Let's look at just three.
John the Baptist is in prison and things aren't going the way he thought they would. He sends a message to Jesus that questions whether He's really the Messiah. Does Jesus say to him, "Now John...just believe!" No, instead Jesus tells him, "Go and tell John what you hear and see: the blind receive their sight and the lame walk, lepers are cleansed and the deaf hear, and the dead are raised up, and the poor have good news preached to them" (Matt. 11:4–5). Jesus references the Old Testament prophecies that characterize the Messiah so John would have "evidence of things not seen" concerning Jesus' identity.
Another example is where Jesus tells a paralytic his sins are forgiven. This irritates the religious elite to no end because only God forgives sins. Jesus knows what they're thinking and asks them: "Which is easier, to say to the paralytic, 'Your sins are forgiven,' or to say, 'Rise, take up your bed and walk'?" (Mark 2:9). Obviously the former is easier because there's no way to empirically verify that someone's sins are forgiven. So here's what happens next: "'But that you may know that the Son of Man has authority on earth to forgive sins,' — he said to the paralytic — 'I say to you, rise, pick up your bed, and go home'" (Mark 2:10–11). Jesus gives them the "evidence of things not seen" (the man's sins being forgiven) by doing the more difficult, here-and-now work of healing the man's infirmity.
Third and last, Jesus makes the claim that those who believe in Him will have eternal life (John 3:16), but what "evidence of things not seen" does He give us to back up His statement? Answer: His resurrection, which consists of much more than His body simply vanishing and believers left hoping He was right. Instead, we read that "To these He also presented Himself alive after His suffering, by many convincing proofs
, appearing to them over a period of forty days and speaking of the things concerning the kingdom of God" (Acts 1:3, my emphasis). 
So in the end, we see that both Boghossian's claim of faith being an epistemology and his distorted / invented redefinitions of faith fall flat, and do so with quite a thud. Instead, a good dose of investigation and apologetics spell out yet again something philosopher Alvin Plantiga has written to those who hold to the Christian worldview: "Perhaps the main function of apologetics is to show that...[we] have nothing whatsoever for which to apologize." 
1. These are fine questions to ask and deserving of good answers. One thing Boghossian is correct about is that we should think critically about beliefs and have valid reasons for believing what we do.
2. Peter Boghossian, A Manual for Creating Atheists
(Pitchstone Publishing, 2013), Kindle Edition, Chapter 2.
3. Daniel Shepard, "Faith is not a virtue
4. I will not broach the subject in this article, but Boghossian's incendiary language is very dangerous and can easily be classified as hate speech. History is replete with examples of various atheist regimes "eradicating" faith by eradicating the people who held that faith.
5. Geisler and Feinberg, Introduction to Philosophy
(Grand Rapids: Baker, 1980), pp. 103-117.
6. Tom Gilson shows Boghossian's error here by pointing out that one may be incorrect about a truth claim, but that is not the same thing as "pretending" you know something that you don't. For Gilson's complete rebuttal of Boghossian's book, see: Peter Boghossian, Atheist Tactician
8. See one explanation for the difference from former detective J. Warner Wallace here: "There's a Difference between Evidence and Proof
9. The Hebrew and Aramaic Lexicon of the Old Testament (HAL).
10. David Hume, An Enquiry Concerning Human Understanding
11. Whether you believe the Bible or not, the above examples show that faith, as showcases in the Bible, is not "belief without evidence." In this case, as Gilson points out in his book, it is right and proper to use Biblical examples to demonstrate Boghossian's error.
12. Alvin Plantinga, "Christian Life Partly Lived," in Philosophers Who Believe
, ed. Kelly James Clark (Downers Grove, Ill.: Intervarsity Press, 1993), p. 69 as quoted by Alex McLellan in A Jigsaw Guide to Making Sense of the World