"Again, the kind thing is to bring the sure Word of God, with its infallible promises, alongside the grace they find exists in their changed hearts. The Spirit will then enable believers to say with assurance, 'I am a child of God and will be forever' This great comfort and encouragement does not come through a private revelation of the Spirit (a witness of the Spirit apart from or in addition to the Bible). Assurance is effected not by imparting new revelation to a person's heart, but by applying what is already revealed in Scripture, namely, the truth that believers shall be saved." (p81)What is Metzger saying? I have been told many times in life that after you "lead someone to the Lord", the next thing you absolutely must do is "give them assurance of their salvation." So you underscore to them that when they begin to doubt this salvation, they should remember that they asked Jesus to save them at this point in time. The eye-opening thing (at least for me) is that Metzger is saying that assurance is not our job.
Metzger goes on to point out that God wants us to be examining our lives to make sure of our calling. "He makes us restless, even to the point of questioning our salvation, so that we may not presume on his favor, but instead, relish his grace." Any confidence we have should be based on the finished work of Christ alone as revealed to us in the Word of God alone. The Bible makes it clear, though, that a result of doing that (placing confidence in the Gospel on a daily basis), will result in a changed life--specifically, patterns of sin in our life will be broken.
"Our eternal security should be focused not on remote past actions but on our present attitude toward Christ. Just as earthly parents can expect physical growth in their children, so we can expect to see a gradual change in the lives of God's children." (p81)