Salvation - God's Work and our Part in It
The direct cause of salvation is always the Lord's work and the Father's decision to give the saved to His Son. He gave us to Him before the World began (John 6:39 combined with Ephesians 1:4). That's why our work to tell the World the Gospel is not the cause of the salvation of anyone, although God allows us to be a part of His work by giving us the job of spreading the Gospel, so those whom He is saving will know the things we must believe and that thing we must do in order to be saved (1 Corinthians 15:3-8 and Romans 10:9, 10:10, 10:13).
From our point of view, there is a trap that makes us think that our will and work guides people into salvation, but it's really God's will and work that does it. We know that it is really God's will and work, because the Son didn't lose any whom the Father gave Him (John 6:39 again). The fact that it is God's will and work is the opposite of this wicked thought that it is our will and work that *directly causes* people to believe and confess Christ and be saved. God allows us to help, because He is gracious to us in allowing us to have a part in His work, but His work wouldn't lose even one, whether or not we help Him by loving them. Even our preaching of the gospel is something that God actively sends preachers to do, and no preacher can resist God when He actively sends, so salvation is God's work primarily, and our work optionally, and the one God predestined to be adopted by His Son Jesus Christ (Ephesians 1:5) cannot resist Him, either.
Another little argument is that if it was our work to cause others to be saved or our brilliance or strength or soul to find salvation in Christ by ourselves, then we'd certainly lose some potential Christians and sometimes potential Christians would not find the strength or brilliance or soulish aptitude to find salvation in Christ. Therefore, it is not our work, because not even one will be lost.