Nothing drains spiritual energy like fear. When believers are repeatedly worried about their salvation, anxiety can cloud their thoughts and distract them from God's purpose for their life. Furthermore, it robs them of the peace and the joy that the Lord has promised his followers. There are several reason why some Christians struggle with doubts about whether they are saved:
Sin. Salvation brings forgiveness and a righteous standing before God. But when we focused on our sins and failures, we may doubt that God could forgive us.
Emotions. Sometimes we rely on our feelings, rather than the truth of God's words, to determine our salvation. Immaturity. Due to the ignorance of scripture or the slow process of change, new believers may begin to question whether they are truly saved.
Legalism. Some Christians evaluate their eternal security by their performance. If they fall short of a standard they themselves set, uncertainty can take root.
1st John verse 3: 19 says we can know that we are of the truth and assure our heart before God. The word assure means to Pacify and calm our soul so we're not consumed by fearful doubts that prevent us from enjoying our new life in Christ.