Christianity must be chosen.
(As fallen Men) we start out turned-away-from God - and must choose (must will) to turn toward Him.
That means the default state is not choosing to become a Christian.
Over time, the consequences deriving from not choosing to become Christian will accumulate in the world; and this affects later choices, making it less likely that people will choose Christ.
This is the direction of history, and the trend of history; and why - at some point - history will come to an end: when no more people can or will turn to God.