How do we know that sanctification is vital for believers?

Sanctification is fundamentally important because it is directly aligned with the will of God for His people, as expressed in 1 Thessalonians 4:3. The process of sanctification involves both internal changes, brought about by the Holy Spirit, and external actions that demonstrate holiness in life. It is through the sanctification process that believers can rightly reflect God's character and maintain a lifestyle that is pleasing to Him. This includes resisting prevalent cultural practices, such as fornication, which were common in the surrounding societies. By adhering to God's call for sanctification, believers actively distinguish themselves as followers of Christ and live out their faith authentically.