Thomas Henry Huxley was an English biologist who was nicknamed "Darwin's Bulldog" for his staunch support of Darwin's theory of evolution. Huxley is also credited with coining the term "agnostic". Following in his footsteps, his grandson Julian Huxley wrote the following about when a person should assume a position of agnosticism:
"I believe that one should be agnostic when belief one way or the other is mere idle speculation, incapable of verification; when belief is held merely to gratify desires, however deep-seated, and not because it is forced on us by evidence; and when belief may be taken by others to be more firmly grounded than it really is, and so come to encourage false hopes or wrong attitudes of mind."
Huxley felt that "all our life long we are oscillating between conviction and caution, faith and agnosticism, belief and suspension of belief." 
The word "agnostic" broken down is "a" for "no" and "gnostic" or "gnosis", which is the Greek noun for knowledge in general. Thus, the word literally means "no knowledge". A formal definition of Huxley's "agnostic" term today where the subject of God is concerned is: "a person who holds that the existence of the ultimate cause, as God, and the essential nature of things are unknown and unknowable, or that human knowledge is limited to experience." 
From this description, it can be said that an agnostic's position is one where they say they do not know if God exists. Speaking more broadly, some agnostics state that it is difficult to hold any truth with certainty.
Types of agnosticism
Agnosticism typically takes one of two forms: hard and soft. The hard agnostic
says that a person can't know anything for sure. Sadly, this is a self-defeating position as the hard agnostic says that they know for sure that they can't know anything for sure.
In contrast, the soft agnostic
says he doesn't know anything for sure. The real struggle for him is not the lack of human ability for knowing a particular truth, but rather how a truth claim can be verified or shown to be true. It is the ancient pursuit of philosophy's study of epistemology: How do we know, and how do we know that we know?
When the issue of determining the existence of the Christian God is added to the mix, things get even stickier.
But perhaps that doesn't need to be the case. What if a person truly follows and applies Julian Huxley's criteria for determining when to be agnostic about a particular truth claim? What would be the end result when Huxley's measures are applied to the claims of the New Testament, and specifically its account of Jesus Christ?
Huxley's first condition is that a belief cannot be mere idle speculation or be incapable of verification.
This first standard seems reasonable, as pure conjecture or hearsay should not be a basis for committing oneself to a belief. The second condition appears logical also and is sometimes termed the principle of falsification, which was used by philosophers such as Anthony Flew in his initial writings on religion.
How do the claims of the New Testament and Christianity hold up under Huxley's first criterion? When the legal/historical methods for determining truth are applied to the New Testament, it stands very firm under Huxley's standard.
The writers of the New Testament never state that their beliefs were based on hearsay or were events that could not be authenticated. Quite the opposite! Apostles such as Peter say, "For we did not follow cleverly devised tales when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty" (2 Peter 1:16). The disciples recorded occurrences that happened in actual space/time, saw these events with their own eyes, and recorded Jesus' life, death, and resurrection so that others would know the truth of what happened.
In terms of falsification, the apostle Paul gave the enemies of Christianity a single truth claim that, if proven untrue, would crumble and destroy Christianity in an instant: "But if there is no resurrection of the dead, not even Christ has been raised; and if Christ has not been raised, then our preaching is vain, your faith also is vain" (1 Corinthians 15:13–14).
What Paul is saying is that if the resurrection of Christ did not occur, then the Christian faith is literally "useless" (vain). That is how Christianity can be falsified: find the body of that Jewish carpenter, and the Christian faith is undone.
But earlier in that same chapter, Paul actually challenges his readers of that day to go check for themselves that the tomb of Jesus was truly empty: "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" (1 Corinthians 15:5–8).
Scholars agree that Paul is literally asking the Corinthians to verify his claims with many others (over 500) alive at that time which saw Christ and could act as witnesses to validate the fact that Jesus' resurrection actually occurred in their recent history.
But, given that we cannot do that today, how can modern day people know that Paul and the other apostles were telling the truth? The apostles answer that question through their grave markers. Many were martyred for their testimony. While people may be deceived and die for a lie, no one dies for what they know
is a lie. All the apostles had to do to save their lives was recant their testimony and say they didn't see Jesus alive—but none did. Greater evidence for believability cannot be had.
Conditions 2 and 3
Moving on from Huxley's first criterion brings the discussion to his second and third standards, which are nearly identical in nature. Huxley says that a belief should be discarded if the sole purpose is to satisfy some psychological desire. He further holds that if the belief is not well-grounded from a "reality perspective", false hopes may be manufactured in the mind. This benchmark measure for a belief is certainly rational, as the only reason to believe anything is because that particular thing (or claim of truth) is true.
Oftentimes, the psychiatrist Sigmund Freud is quoted to show how religion fails such a test. He says, "[Religious beliefs] are illusions, fulfillments of the oldest, strongest, and most urgent wishes of mankind.... We call belief an illusion when a wish-fulfillment is a prominent factor in its motivation, and in doing so we disregard its relation to reality, just as the illusion itself sets no store by verification." 
However, Freud's criteria do nothing to prove or disprove God for his sword cuts in both directions. Could it not be true that the atheist/agnostic has wishes and urges of their own? Perhaps their wish is that a God who will call them to account for their actions does not exist? Such a desire can be very motivating and drive a person to hold an atheistic/agnostic position. Freud's words have no power whatsoever to determine if the truth claims of Christianity are valid or not.
Freudian thoughts aside, how does the New Testament stand up against Huxley's second and third standards? As it does with Huxley's first measure, the New Testament does extremely well.
First, from a legal or historical perspective, no other antique document comes even close to the New Testament in passing the general criteria for judging the validity of a historical work.
The New Testament passes the bibliographical test (manuscript reliability and early dating), internal evidence test (multiple key testimonies all of which match), and the external evidence test (outside evidence that corroborates the document's testimony)—all with flying colors.
Second, the New Testament is not written as though it is a lie.
The New Testament writers would not have invented accounts such as Jesus being buried by a member of the Sanhedrin, women being the first witnesses of Christ's resurrection, and other such things. They simply go against what deceptive writers would have wanted to portray.
Instead, what is found is a strong commitment to accuracy no matter where the evidence led them. Such dedication is seen in the pen of Luke: "Inasmuch as many have undertaken to compile an account of the things accomplished among us, just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the word, it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; so that you may know the exact truth about the things you have been taught" (Luke 1:1–4).
Lastly, as has already been pointed out, the New Testament writers died for their testimony.
As theologian and professor Peter Kreeft points out: "Why would the apostles lie? ... If they lied, what was their motive...? What they got out of it was misunderstanding, rejection, persecution, torture, and martyrdom. Hardly a list of perks!" 
The treatment Kreeft lists certainly is not desirable from a psychological perspective and would produce no false hopes in the disciples, as they would obviously know their claims were false if they were lying. Adding this to the previous arguments, we see that the New Testament accounts overcome Huxley's second and third hurdles for being agnostic.
So, should you be agnostic?
In the end, a person who claims to be agnostic about Christianity, but uses Julian Huxley's own criteria for determining whether one should be agnostic, will have to seriously reconsider their position.
With the hard agnostic viewpoint being ruled out as self-defeating, and the soft agnostic viewpoint being challenged by the compelling evidence of the New Testament, the more reasonable conclusion for the agnostic to reach once everything has been examined seems to be that Christianity is true. 
 Julian Huxley, Religion Without Revelation.
(New York, NY: Mentor Books, l957) p.l7.
 Dictionary Definition of Agnostic
 Sigmund Freud, The Future of an Illusion
, (New York: Anchor Books, 1964), pgs. 47,49.
 Peter Kreeft "Why I Believe Jesus is the Messiah and Son of God" in Why I am a Christian: Leading Thinkers Explain Why They Believe
, Geisler & Hoffman, ed. (Grand Rapids: Baker, 2006), pg. 250
 For more information on the reliability and historicity of the New Testament, see Dr. Mark Roberts (Ph.D. New Testament Origins, Harvard) excellent work Can We Trust the Gospels?