Dear Mr. Green --
I also once ceased to believe and told others there is no God. In fact you and I both lost our faith and then returned to it. But surely we did not return on our own steam? Surely we were recalled by God? For no man can come, nor come back, to God unless God sends for him. The grace He has thus shown us for a second time is the proof that He has forgiven us. He has not cast us aside even though we, for a time, cast Him aside.
Don't forget that Bunyan, as he himself tells us, thought that he had at one time committed the unforgivable sin. Yet he lived to write the Pilgrim's Progress and to be a great Christian champion.
It has always puzzled me very much that Our Lord should have told us there is an unforgivable sin and yet not told us what it is. If it is a particular act which could be done at a particular time, the warning does not seem to be any use -- like being told that there is a poisonous vegetable but not told which it is. But it may mean persistence in ordinary sin, a final refusal to repent or even to try to reform. If so, the warning would then be useful, a reminder that there is a point beyond which return becomes impossible. His purpose would be to fix the danger in our minds, but certainly not to set us looking through all our particular sins and trying to guess if some one of them was It.
As far you (and I) are concerned I have no doubt that the fear you mention is simply a temptation of the devil, an effort to keep us away from God by despair. It is often the devil working through some defect in our health, and in extreme cases it needs a medical as well as a spiritual cure. So don't listen to these fears and doubts* any more than you would to any obviously impure or uncharitable thoughts.
With hearty good wishes.
C. S. Lewis
*Of course, like other evil temptations, they will not be silenced at once. You will think you have got rid of them and then they will come back again -- and again. But, with all our temptations of all sorts, we must endure this. Keep on, do your duty, say your prayers, make your communions, and take no notice of the tempter. He goes away in the end. Remember I John iii, 20 'If (=though) our heart condemn us, God is greater than our heart.' Let us sometimes pray for one another.
(Collected Letters, Volume III, pp. 1340-1341.)