Prayer does two things. It shows the complete sufficiency of God and the complete helplessness of man. It shows that God is not lacking, that He is in need of no thing, that He is infinitely and gloriously wealthy, that He can give to, He can bless and He can answer without the need of help from anyone or anything else. And it also shows that we are in desperate need of that kind of sufficiency.
Heaven is not a place for those who are afraid of hell; it’s a place for those who love God. You can scare people into coming to your church, you can scare people into trying to be good, you can scare people into giving money, you can even scare them into walking down an aisle and praying a certain prayer, but you cannot scare people into loving God. You just can’t do it.
If you add to or subtract from the cross, even if it is to factor in biblically mandated religious practices like prayer and evangelism, you rob God of His glory and Christ of His sufficiency.