I’ve asked my priest about the Te Deum, and he hasn’t been very helpful.
Haydock’s Catholic bible commentary has this to say on 1 John 1:8.
Ver. 8. Not that we say or pretend we have no sin; thus truth would not be in us, and we should even make God a liar, who has declared all mankind guilty of sin. We were all born guilty of original sin; we have fallen, and still frequently fall into lesser sins and failings. We can only except from this number our Saviour Christ, who, even as man, never sinned, and his blessed Virgin Mother, by a special privilege, preserved from all kind of sin: and of whom St. Augustine says, “that for the honour of our Lord, when we speak of the holy Virgin Mary, he will have no mention at all made of any sin.” (Witham)
Augustine also said:
He, moreover, who says that any man, after he has received remission of sins, has ever lived in this body, or still is living, so righteously as to have no sin at all, he contradicts the Apostle John, who declares that If we say we have no sin, we deceive ourselves, and the truth is not in us. 1 John 1:8 Observe, the expression is not we had, but we have. --St. Augustine, On Man’s Perfection in Righteousness.
But in the Te Deum we ask God to vouch safe to keep us this day without sin.
Does that mean without any sin at all?
And if that is what we mean, why doesn’t God ever seem to answer the request?
And if it isn’t possible for Him to grant such a request immediately, why do we ask?
And why do we specify a time (this day)?
Aren’t we supposed to pray in faith, and how can we do that if we ask the impossible?