“But we should always give thanks to God for you, brethren beloved by the Lord, because God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth. It was for this He called you through our gospel, that you may gain the glory of our Lord Jesus Christ.” 2 Thessalonians 2:13-14
We are born-again by God’s Spirit when we believe in Christ’s atonement for our sins. That is salvation - a singular decision with eternal permanence. We are also sanctified by the same Spirit of God. But unlike salvation sanctification is a lifelong process.
In his book The Pursuit of Holiness, Jerry Bridges describes the joint venture of holy living. “...The farmer knows that unless he diligently pursues his responsibilities to plow, plant, fertilize, and cultivate; he cannot expect a harvest at the end of the season. In a sense he is in a partnership with God and he will reap its benefits only when he has fulfilled his own responsibilities. Farming is a joint venture between God and the farmer. The farmer cannot do what God must do and God will not do what the farmer should do. We can say just as accurately that the pursuit of holiness is a joint venture between God and the Christian. No one can attain any degree of holiness without God working in his life; but just as surely, no one will attain it without effort on his own part.”
“For God has not called us for the purpose of impurity, but in sanctification.” 1 Thessalonians 4:7