No one really looks at it like this unfortunately, because they claim all thee essentials of the Gospel. The problem is, so do Mormons, and Catholics, but these 2 "faiths" are considered heretical to any protestant.
Should we just let them continue in their unbelief and fallacy than? No, absolutely not. Being graceful towards them would be understanding they are in false doctrine, need to repent, and believe on the Gospel. By ignoring this fact, or top shelving it, we are neglecting them and whats scarry is that we allow then to continue in this, and/or would be willing to fellowship with them as they're our brothers and sisters.
Unacceptable. They need the Gospel and we are called to preach it to them. No one is saying that they are or are not elect. Faith comes by hearing the Word, and by the Word of Christ. Without holding true to this very fact we are neglecting them of the saving power of God. Would you allow a Mormon or Jehovah Witness' to pass by you and preach their version of the gospel to your friends or family? Not unless you're spineless. The same thing goes here.