We can know we are truly saved when we recognize our utter dependence on Christ and see Him as our sole source of righteousness.
The assurance of salvation stems not from our works or feelings but from the finished work of Christ and our faith in Him. In John 6:37, Jesus assures us that all whom the Father gives Him will come to Him, and those who come to Him He will never cast out. Our awareness of being undone, as stated in Isaiah 6:5, leads us to seek mercy and grace from the Lord. It's through this posture of humility and recognition of our sinfulness that we can rest in the promise of salvation provided through Christ's propitiation for our sins.
Scripture References:
John 6:37, Isaiah 6:5, Ephesians 2:8-9, Romans 4:5, Matthew 7:21-27, 2 Corinthians 13:5
Commentary