Pros: When done right, everyone will be vastly smarter, kinder and more beautiful.
Cons: When done wrong, everyone ends up living in the dark ages again due to only allowing hardcore christians to breed. In addition, avoiding a genetic bottleneck - which would lead to greater susceptibility to diseases, both genetic and pathogenic - would be very difficult.
This may go as far as rendering humanity unable to evolve to face a different climate in the future, though with sufficient advances in genetic engineering, that could probably be avoided.