This text captures the essence of believing in Christ. It is not any of our work, but God's work within us. It is not because of our worthiness that we are saved. It is not that God knew before that we would have faith, so He saved us. Our salvation is based on nothing, I repeat, NOTHING, that we can do. God, by His Holy Spirit and through the preaching of the Word, works 'a good work' [faith] in us.
And what's more is that since God has begun this work in us, He WILL complete it. God will not allow those whom He has effectually called unto salvation to fall away from His grace, but they shall persevere until the end. Until the day of our Lord Jesus Christ! Those who try to live a Christian life in their own abilities will fall away (Matt 13:20-22). But those who show themselves to be regenerate by God in their walk of holiness and love for their neighbour can have the comfort that they are secure in Christ. Although they may backslide and fall into sin, the Holy Spirit convicts them of their sin and compels them to repent and seek to live a life free from sin.
Now this does don't make the believer slack in living a holy life, as though he has a guaranteed safety net that no matter what he does, he will never fall from God's grace. Instead, by knowing that God has pledged to keep him safe forever, the believer desires more and more to please God through a humble and grateful attitude.
What a comfort that is for believers, and at the same time it reminds us to devote our lives to the One who has saved us.
"Now may the God of peace Himself sanctify you completely; and may your whole spirit, soul, and body be preserved blameless at the coming of our Lord Jesus Christ. He who calls you is faith, who also will do it." (1 Thess 5:23,24)
