Some of it is:
Since Darwin, we don't need the Christian explanation for Man.
Cultural relativism - if Christianity is right, then how come so many cultures disagree?
Morals change over time - if Christianity is right, then how come what is good changes?
Atheists are often decent people - well behaved, hard-working, smart etc.
All Christians are hypocrites; some are evil hypocrites; some are dumb or crazy as well - clearly Christianity doesn't work (or, not well enough).
When a society gives-up Christianity there is no instant catastrophe. Things keep going.
This world is obviously real, the next world is not.
Historical knowledge about what happened thousands of years ago in Palestine (etc) is too insecure, to debatable to pin your life upon. (We don't really know anything for sure.)
Miracles can't happen - therefore there is always an explanation, if not a scientific explanation then some psychopathology (humans are very suggestible, and unreliable witnesses) - miracles are not evidence for anything.
Prophecy can't happen - therefore prophecies are like horoscopes - vague predictions which people imagine to have been fulfilled because they want then to be true.
Religions are obviously explainable in terms of socio-political control and manipulation.
Religions did a lot of evil things - we need to find something better.
And so on...
My point here is that Christianity is the truth and all the above are wrong; but in real life there is zero possibility of taking objections such as these above, one at a time, and refuting them...
- not least because the people who hold such opinions will resist the process, will find it boring (bore-ing), will find it aggressive or condescending, have other things they want to do more or which they consider more important.
(Seculars regard Christians as salesmen - and they exhibit 'sales resistance'.)
What we are dealing with, is a total difference in perspective - dominant, mainstream, Leftist secularism is a kind of inversion of Christianity - not a perfect and coherent inversion, not yet, but one which is becoming more-perfect and more-complete over time - and is now become so seemingly-autonomous from the Christianity against-which it reacted, that many modern secular intellectuals have come to believe the nonsense that Leftism is an extrapolation of Christianity, rather than its inverse!
What this means is that Christian evangelism must be tough, simple and robust-enough to get-by and get-through this veritable meteor shower of bad but distracting anti-Christian pseudo-arguments.
There may only be a small opening for a few moments.
What is needed is somehow (and there is no formula) to get the Christian perspective across there and then, encapsulated into a vision - to show people, however briefly, that there is a totally different (better) way of looking at things, looking at reality from a different direction and with different basic suppositions - from which all the above quibbles are dissolved in an instant.
Everything clicks, or rather everything opens-out - in an instant, over a few seconds...
Of course this vision can be, and often is, rejected subsequently (Christianity is always and necessarily a choice) - but to induce such a vision is the aim and hope; and a vision can be strong, and will inevitably tend to grow (if it does not fade or become corrupted).
So, at least some Christian apologists and missionaries need to specialize in depicting visions: they need to be practical visionaries.