John MacArthur has been going through a very similar sermon series that I've been listening to on the radio. So I've been thinking about this topic a lot. MacArthur's mostly focusing on how modern English translations have downplayed the Bible's use of the word "slave" to describe salvation. Also how most modern evangelistic movements intentionally omit any references to making Christ the Lord of our lives, committing our lives to Christ, acknowledging our sin, and even repenting of our sin. Because they believe it is more important to get people saved and sealed and then release those same people to go live out their lives however they see fit with absolutely no acknowledgment of GOD after that initial decision. Preferably they would make GOD the Lord of their lives at some point in the future, but it's not really necessary because they're saved and, even if they leave the faith or deny the existence of GOD altogether, they're still going to Heaven because of that one decision made in a moment of time. That's a complete and total prostitution of GOD's Word and it bears no resemblance whatsoever to the true Gospel of Jesus Christ.
If someone refuses to make Jesus the Lord of their life - in other words, they have to give up EVERYTHING to Christ: their family, their possessions, their money, their job, their property, their spouse, their body, their mind, their soul, their spirit, their free will, their future plans, etc.; and become His slave for eternity - then they can't be saved. GOD doesn't accept part-time commitment (or Sunday-morning Christians), it's all or nothing.
I think if Charles Spurgeon was disturbed by the direction the Church was taking in the 1800s, then he would be absolutely horrified by what the Church has become in the 2000s.