"Work for your own sake with fear and trembling : for it is God who works in you and willing and doing it, according to his pleasure." Philippians 2. 12, 13
Some believers believe that spiritual growth is automatic after the new birth. It's an illusion.
The Apostle Paul asks the Philippians to work for their own salvation. Would it be to make efforts to ensure the salvation of his soul ? No, Paul speaks to children of God who know that they can not add anything to the work of Christ to save them, a perfect, unique, definitive work. At the new birth, God gives us the gift of eternal life. How could he take back what he gave us by the sacrifice of his Son ?
In the verse of the day, the Apostle Paul's message to the Philippians is to make every effort to promote their spiritual growth, as when cultivating their garden. The apostle Peter has the same message : "Rejecting therefore all wickedness, all deceit, hypocrisy, desires and slanders of all kinds, longing, like new-born children, the pure milk of the Word, so that you grow by him to salvation, though you have tasted that the Lord is good" (1 Pet. 2-3). It requires an effort of love, a personal effort, freely consented. God produces "will and do", desire and action ; but we are not passive in his hand. God desires that we engage our hearts for him, that we become spiritually mature, experienced men and women : "Watch, hold fast in the faith ; be men, strengthen yourself" (1 Corinthians 16:13).
To grow spiritually requires time, energy, will, and above all, a fervent love for the Lord. This growth is not instantaneous, but progressive. Spiritual maturity is not reserved for a few. It is within our reach, if we seek it : "Practice piety" (1 Tim 4: 7). The athlete trains to reach the highest level ; the musician exercises to perfect his art, etc. Likewise, spiritual maturity is achieved through the practice of a lifetime with Jesus Christ.
However, do not confuse spiritual growth with the acquisition of biblical knowledge. True spiritual growth is reflected in the quality of our behavior towards God and toward men.
Source (Pleasing the Lord)