It's one of the silliest cliches on the internet! Find a popular Christian page on a social media platform, like Facebook, look for a post that says something about God, and look through the comments section. In spite of the very obvious Christian nature of the page, you are virtually guaranteed to find at least one atheist asking "which God?"
The irony is, in another context, this would be a semi-reasonable question. An atheist, studying on his own, may well come across non-specific arguments, such as the Cosmological Argument, and conclude that there must be a god after all. The Cosmological argument establishes the necessity of a god, but does not necessarily show us which god. Thus, our now ex-atheist, with no other person in view, may logically ask "which god?"
But the trolls who go to Christian pages only to ask "which god?" are not in such a position of wisdom. They are not genuine Theists seeking to figure out which god is the real God, but remain in their atheistic state, attempting the divide and conquer tactic. With atheism being, as Isaac Newton once put it, "...so senseless and odious to mankind that it never had many professors", atheists instead resort to trying to get people of different religions to fight each other. More often than not, they will take the two biggest faiths, Christianity and Islam, and simply ask "how do you know Christianity is true and not Islam?"
But realistically, you should be able to stop at "how do you know Christianity is true?" Why does Islam need any kind of seat at this table? You're not a Muslim, so you don't need to know any specific reason Islam is false, and I'm not going to waste time explaining it to you. If you want me to explain why Islam is wrong, convert to Islam.
With that being said, the simplest reason I know Christianity is true and Islam is not is because I know Christianity is true. See, of all the religions in the world, at least as far as I have studied, Christianity is the most intellectually robust. Most notably, our God actually showed up!
Throughout history, God has entered the world in various ways, from spectacular sights such as pillars of fire and smoke guiding the Jews through the wilderness, to humble appearances, like a babe in a manger. With such a comprehensive historical record, I can say, with almost complete certainty, that Jesus Christ lived a miraculous life, was very publicly murdered, and was seen alive again by many witnesses. Witnesses who were willing to die for their testimony. Now, you tell me, how many other gods have stepped into human history, literally shaping it to the extent where our own calendars are based on His birth?
More importantly, how many other prophets walked out of their grave? Muhammad is dead. Buddha is dead. Guru Nanak is dead. Joseph Smith is dead. Charles Darwin is dead. Even Moses, God rest his soul, is dead. The founders of every major religion on this planet, and indeed every religion not founded within the life expectancy of today's human being, are all dead. We even have a rough idea of where most of them are buried.
But where is Jesus? Unlike all these other prophets, having been brutally murdered and laid in a tomb, Jesus appeared alive and well, with little evidence of His crucifixion aside from the holes where the nails were hammered, and the slit in His side from where He was pierced with a spear. Our God lives! 2,000 years after our religion was founded, our main prophet sits at the right hand of the Father until His enemies are made His footstool! "Which God?" That one! The one who spoke the world into existence over a period of 6 days. The one who cursed the very ground for our rebellion. The God who ushered Noah into the ark, protecting him from the flood that placed marine life on top of mountains, where we still find their fossilised remains today. The God who heard the oppressed cries of His people, tearing the nation of Egypt apart to free them, and used their tiny military force to overthrow the promised land. Most importantly, the God who entered the world through the virgin womb of a humble maid, lived a perfect life, ran a miraculous ministry, died for our sins, and rose again so that everyone who confesses Him as Lord, and believes He rose from the dead, will receive eternal life. That's which God.