Christians claim that God is all powerful, all knowing, loving and the creator of the universe.
An all knowing deity would know of a universe where all his aims for the people of the universe would be achieved without any suffering.
An all powerful deity would be able to create such a universe.
A loving deity would want to create such a universe where there is no suffering of his creations.
But in our universe there is suffering, not only of adults, who you might argue have free will to "choose" to suffer, but also of children and babies that do not have the ability to make that choice. In fact we see small children and babies suffering due to "acts of God".
Hence our universe can not have been created by an all powerful, all knowing, loving deity.
Hence God can not exist.