General Arguments Against Faith
In "Is Your Religion True" James essentially makes two arguments against belief in God: (1) lack of consensus and (2) lack of ubiquity.
Lack of Consensus
Frankly, this is a poor argument in general.
- Especially during an election season, it is clear that people are not always guided purely by reason and in many cases may not even devote sufficient time and consideration to even some of the most important issues to make a well informed decision.
- If consensus is so important and inevitable, James should be wondering why only 2.32% of people in the world are atheists instead of why only 33.32% of people are Christians. 
That said, James advances two more specific arguments in this vein:
Religious Denominations vs. Scientific Consensus
James argues that major religions, such as Judaism, Christianity and Islam have splinter groups numbering in the hundreds and perhaps thousands whereas the scientific community has relative consensus. This more specific argument is in error because of the reason above, but is flawed in two other ways as well.
First, by focusing on the broad range of disagreements, ranging from major doctrines such as the Trinity to relatively minor disagreements over church government, and ignoring large areas of consensus, such as the existence of God, the existence of good and evil, etc., the argument presents a deeply divisive view of religious people. Conversely, by attempting to focus on the relatively narrow range of natural laws and focusing on large areas of consensus, such as General Relativity and Biological Evolution, and ignoring areas of disagreement ranging from possible unification theories and forces behind evolution (leaps, gradual, divinely directed), the argument presents a highly skewed comparison.
Second, to skew this comparison even more, it divides religious people according the general public, but focuses only on a highly focused group of people when considering views on science. Notice the words used. When speaking about religious people, the claim is that "the world should be moving towards some kind of consensus with regards to the true and proper religion" whereas the relevant people related to scient are "the scientific community".
Attempts to support this comparison by remarking that trained religious leaders share no such consensus are also misleading. Not only is the claimed consensus of scientists exaggerated, but unlike scientists who receive a more unified education, religious leaders are generally trained according to their particular sect.
The second argument James attempts to make to support the idea that any true religion would inexorably achieve universal consensus is that God is a much better communicator than we are and essentially should be able to convince us all that He is right. Basically, the claim is that God's problem is a presentation problem, that if He really existed and wanted us to believe, He could perform a sign or present the right reasons for us to believe. However, this assumes that everyone would approach God's presentation rationally and fairly. Not only do people frequently act irrationally, it seems clear that many people do not want to believe everything God might have to say. Acceptance of our own failings, moral pronouncements, thou shalts and shalt nots, the acknowledgement of a higher power who cares what we do, etc. As a result, it is unsurprising that many people disagree on the truth and would continue to do so even if God tried to convince them.
In the parable of the rich man and Lazarus, the rich man and Lazarus, a poor beggar outside his door both died. The rich man went to hell and Lazarus went to Paradise. Seeing Lazarus in Paradise with Abraham, the rich man begged Abraham to send Lazarus back to bring the truth to his brothers. Abraham gave the following reply:
Abraham saith unto him, They have Moses and the prophets; let them hear them. And he said, Nay, father Abraham: but if one went unto them from the dead, they will repent. And he said unto him, If they hear not Moses and the prophets, neither will they be persuaded, though one rose from the dead. - Luk 16:29-31
The second argument James presents is much more powerful. The argument is essentially that if there is a God who is not a respecter of persons and desires to reach out to mankind in general, He would not leave gaps in the availability of the truth. Two prominent examples of religions that readily claim such gaps are Mormonism and Islam, which basically claim that God's original revelations in the Bible were corrupted and that after hundreds of years, He finally decided to set the record straight.
This is a huge problem for these religions because it clearly implies that God simply abandoned the world to spiritual darkness, ignorance, and ultimately judgement for many generations at a time. This argument demonstrates a solid reason to reject any religion that does not at least claim historical ubiquity.
Clearly, James believes this critique extends to Christianity as well. However, it does not for two reasons:
Historical Ubiquity of Special Revelation
James claims that if Christianity is true, it implies that God's message was delivered "at an arbitrary time in human history and at an arbitrary location to a handful of people in just one language". Christianity does not teach that God finally revealed His plan of salvation through Jesus Christ in roughly AD 30. To the contrary, God revealed His plan as soon as it was necessary. That even though we had violated God's righteous standard, separating us from Him, He still loved us. To atone for our sins, Jesus Christ, the divine man, payed the penalty for our sin, allowing us to be reconciled with God for eternity future and that all who looked forward to it were saved, just as we look back and are saved.
Even before pronouncing judgement on Adam and Eve for eating the forbidden fruit, God tells them that a future descendent would defeat the serpent [Gen. 3:15]. It also seems clear that God communicated more than that to them. In Gen. 3:21, we see God making coats of skins for them and in Gen. 4 we see Cain and Abel apparently had at least a basic understanding of the sacrificial system and dealt directly with them. We also see God advising Cain and Cain disregarding God's advice.
By making the first revelation to the parents of the rest of the human race, God not only provided the necessary truth to them, but ensured that at least all mankind had revelation from the beginning of time that could be passed on. This doctrine of ubiquitous revelation over history sets mainstream Christianity apart from some other religions, such as Mormonism and Islam.
Global, Ubiquitous General Revelation
However, as James points out, just having revelation available to some people at all points in time is not enough. It is necessary that some revelation be available to all people at all times. The Bible teaches there are 3 aspects of General Revelation to all men.
First, in Romans 1, Paul declears that the Creation is part of God's general revelation to all men, so that they are without excuse:
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness; Because that which may be known of God is manifest in them; for God hath shewed it unto them. For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: - Rom. 1:18-20
Second, conscience is given to all men, that they may know there is a moral standard and they have fallen short:
For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another - Rom 2:14-15
What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin; As it is written, There is none righteous, no, not one...Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin. But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: For all have sinned, and come short of the glory of God - Rom.3:9-10,19-23
Third, the inner light. The exact function of this revelation is not entirely clear, but it is a clear reference to Christ as the light, which would imply a divine ministry to all people:
That was the true Light, which lighteth every man that cometh into the world. John 1:9
Judging the Ignorant
James anticipates the argument that only those who hear of Christ must accept Him. The rest are judged on relative merit. This is, as he concludes contemptible, for it would transform the good news of the gospel into a message of condemnation for those who hear the gospel and reject it, while giving those who don't hear it a relatively free pass.
Scripture clearly does not teach this position. Jesus is unequivocal on the ways to God:
Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me. - John 14:6
Clearly, there is only one way. But the gospel is very simple:
(1) - There is a God
(2) - We have all sinned, justice demands punishment
(3) - Someone else must take our punishment, that someone, is Jesus Christ
It seems reasonable that most of that message is available to all through general revelation (Creation -> God, Conscience -> sin & judgement). Most people get the third leg from God's Special Revelation in the Bible, whether directly by reading or indirectly through others. However, as James points out there is a lot of time between the coming of Christ (and before that) during which the Americas in particular seem cut off from that source of revelation.
Fortunately, enough revelation was given to Adam and Eve to save them, enough of that may be passed on in cut off cultures to allow enough truth to be clear to those who seek it. Also, not all general revelation is thoroughly understood, particularly the ministry of Jesus Christ as the inner light to all people. The essential point is that if Christianity is true, God has provided sufficient revelation to any person that has ever lived that they can reject or accept and if they accept God can make sure they receive the truth they need in order to come to the Father through Christ.
 CIA World Factbook