In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit.

Eph. 1:13

When I wake up in the morning and all the demands of the day flood upon my soul; I look to the Cross. There I know that I am forgiven, there I am healed, there I am made free from my selfishness and pride, and there I know-I know that I know-I am accepted by God.

The first and fundamental ground of our assurance, because it is the sole ground of our salvation, is ‘the finished work of Christ’.  Whenever our conscience accuses us, and we feel burdened with guilt, we need to look away from ourselves to Christ crucified.  Then again we will have peace.  For our acceptance with God depends not on ourselves and what we could ever do, but entirely on Christ and what he has done for all on the cross.

John Stott, Authentic Christianity (Downers Grove, IL: InterVarsity, 1995), 211.