Modern Christianity does not fulfill biblical signs
The New Testament mentions some of the signs that believers should exhibit. If we assume the Bible is true, modern Christianity does not fit this description and therefore modern Christianity is not based on truth and should not be followed.
The argument is stated by the character Smerdyakov in Dostoyevsky's The Brothers Karamazov:
- "it is said in the Scripture that if you have faith, even as a mustard seed, and bid a mountain move into the sea, it will move without the least delay at your bidding. [...] you try yourself telling this mountain [to move...] You'll see for yourself that it won't budge, but will remain just where it is however much you shout at it, and that shows, Grigory Vassilyevitch, that you haven't faith in the proper manner, and only abuse others about it."
In contrast to mainstream Christianity, the charismatic movement claims that miracles are still routinely performed by believers in the modern age.
"He replied, "Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you.""
"Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father. And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son. If ye shall ask any thing in my name, I will do it."
"If you believe, you will receive whatever you ask for in prayer."
"And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover."
Snake handing has lead to several deaths.  Poison drinking is generally not practiced since it generally is harmful or fatal. Accidental poisonings, which would not put God to the test, occur to believers. We don't see mountains move or any other extreme events as commonly recounted in the Bible. When investigated, faith healing is fraudulent. Apologists claim that prayers are not answered because they are improper requests. This is exactly the point of this argument: no prayers are answered because modern Christians are misguided.
If that were true, it would be observable in demographic data. However, there is no evidence this is the case.
Despite the allegedly historical miracles, perhaps the above claims are only metaphorical.
- "Faith that can move mountains is not meant to imply a faith that can literally move literal mountains. The point Jesus was making is that even a little bit of faith—faith the size of a tiny mustard seed—can overcome mountainous obstacles in our lives. "
Even granting the possibility that the verse is a metaphor, there should be some observable effect of faith and prayer. However, when rigiously investigated, there is not even a modest observable effect. Skeptics ask what is the difference between an unobservable effect of faith and no effect at all?
Perhaps the power of miracles were only intended for the early church: 
- "Those signs were descriptive of the disciples only, not every Christian down through the centuries. "
There are also concerns that the snake handling and poison drinking was a late addition and not part of the original gospel.