"But here someone could still question if any man can ever really know that he's a child of God, or if such assurance ever can or should be sought. This is one of the stubborn excuses usually brought forth by such who still get along best in darkness and uncertainty. But answer: The entire Scripture witnesses, that all the faithful of old have received the testimony that they pleased God by faith (Heb. 11), and only by such faith was it possible for them to suffer the martyr's death in joy. And the apostles plainly say, 'The Spirit itself beareth witness with our spirit, that we are the children of God.' They say, 'He that believeth on the Son of God hath the witness in himself,' That he shall 'know that he has eternal life.' That 'he that believeth not God hath made Him a liar,' etc. (Rom. 8:16, I John 5:10, 13). Note this last one! When the Lord explicitly says: 'He that thristeth,' 'comes,' and he that wants, takes of the 'water of life freely', he that believeth has 'eternal life'; even if his sins should be 'red like crimson,' they shall however be 'white as snow,' etc. But if I am still always uncertain about the forgiveness of my sins, what is that other than to make God a liar?' It's like saying: 'I don't know if what the Lord says can be relied upon; I know not for certain, if God is truthful?' What a beautiful confession this would be for a Christian!
"Therefore, says Luther in the explanation of Galatians 4:6, 'We should from day to day aspire to go from doubt to assurance. And we should take pains to pull up, by the roots and all, the very injurious delusion by which all the world has been seduced, namely, the thought that man cannot know for sure if he is in the state of grace or not. Because if we doubt the grace of God twoard us, and do not think it certain that God has pleasure in us for the sake of Christ, then we deny that God has redeemed us; then we overthrow all His works and benefits He has ever shown us.' Truly, the one, who is satisfied without assurance of the treasure, sets no great value upon it! It is quite another matter, that one should seek assurance in the Word and not in feelings."