How can I dare to approach God with an evil conscience and thank Him for forgiveness?...Only an insane person would say, "Forgive me for what I have done, but I intend to keep on doing it; I will insult you whenever I see you, but please forgive me." Yet that is the way people act who want to take comfort in God's grace and still continue in their sin...
What matterrs is not the external enormity of the sin but the attitude of my heart in connection with that sin. A sudden sin of passion or temper does not extinguish faith, for I sinned without wanting to do it. I can remain in grace. But where there is persistence in sin against conscience and better knowledge, faith departs, one cannot pray to God, the Holy Spirit moves out of the heart, and another spirit moves in.
...All sins are great sins. Even the so-called sins of weakness, of which the justified cannot rid themselves, are no trifle. Even though faith is not extinguished thereby, they are no joking matter.
And let no one find security in the thought that he is one of the elect and therefore bound to be saved, regardless of what he does. If you live in your sins and persist in them, that is a sign that you are not among the elect. Not as though God really did not want you, but He foresaw that you are such a disgraceful scoundrel and that you abuse His grace. No indeed, if you are that kind of person, you are not in the state of grace, and if you persist in this condition you will be damned! No one can deny that Adam and Eve were among the elect. Yet the fell and lost God's image, the Holy Spirit, their holiness, everything. But they repented and thus returned to the state of grace.
(C.F.W. Walther - Law and Gospel from Twentieth Evening Lecture on Thesis X)