I would say that they are covered by the grace of God.
One does not fall from grace over one sin but rather a deliberate rejection of one's Christian faith.
In actual practice I would agree with you that we "don't fall from grace over one sin, but rather than a deliberate rejection of one's Christian faith." But I am trying to dig a bit deeper in this thread: that is - it only takes one sin till one faces the wages of eternal death. And the only payment for sin is a blood atonement. No one will escape the wages of sin unless they are clinging to the blood payment of Jesus on Calvary. Thus one can I suppose say it is God's grace that "covers" one errant sin of a believer - but in actuality a sin must be forgiven, not just "covered" - And so the only way a Christian who sins will get into heaven is if that sin is forgiven by the blood of Christ.
And I conclude that when a Christian sins they are forgiven because of their faith in that blood atonement.
At one point in my life interpreted "confess" in I John 1:9 as saying the right words of sorrow/grief/ et. to God. I thought that if I got angry at my brother, and then that evening in a prayer told God that I was sorry, that then God would forgive me. But this approach makes forgiveness depend upon my external actions (words in fact!). Forgiveness is because of what Jesus has done, not because of what I might be able to do. Furthermore, at least for me, this view made me think God was a tyrannical kind of Judge watching me ongoingly threatening to send me to hell if I did not confess (say the right words).
A key in understanding here is what the Biblical word "confess" means: it means literally = to say the same thing as - or "to agree with". So confession is at the root a heart attitude toward God. When this heart attitude is present, it should and will be expressed in proper words, but the words are not the proof of confession.
Thus in I John 1:9 - "if we confess our sins" does not require the correct external words before God forgives. The requirement is a heart of agreeing with God (believing in the need for His Son's atonement).
I maintain that to understand, teach, and live a proper concept of God's love in forgiving our sins will lead to less sinning, not more sinning. His forgiveness is so great that he does not count the number of our sins, and He forgives, and forgives, and forgives. If it was not so, I would have used up my 490 (70x7) forgivenesses long ago!