Salvation is the work of God. It came through Jesus Christ by the shedding of his blood on the cross (see Jn 3:17; Ro 5:9). Those who experience salvation are given eternal life as a gift of grace that they receive by faith — this gift comes because of God’s mercy, not as a result of human effort.
Since salvation is a work of God’s grace from beginning to end, those who belong to Christ can be assured that they will never lose their salvation. They have been appointed to receive salvation, not to suffer wrath. There is also a subjective dimension to this assurance. The Holy Spirit “testifies with our spirit that we are God’s children” (Ro 8:16). In other words, when we are children of God, the Holy Spirit helps us know with confidence that we are heirs of eternal life.
But assurance is no excuse for complacency. God’s grace transforms those who are in Christ. Therefore, those whose lives show no evidence of this transformation should examine themselves to see if they have genuinely trusted in Christ. For such people the question is not whether they might lose their salvation but whether they ever possessed it in the first place.
Taken from NIV Essentials Study Bible