I have a scholarly book on Martin Luther, and when it comes to the subject of ‘The Church’, there are three chapters on this, respectively entitled, “The Church Territorial”, “The Church Tutorial” and “The Church Ministerial”. You are wise to check what Luther meant by ‘the Christian church’ in context of saying there is no greater sinner than it.
At the start of “The Church Tutorial” we read,
“Visitation had established the outward form of the Church, but Luther
well knew that the Church of the spirit cannot be engendered by the
arm of the magistrate. The true Christian Church is the work of the
Word communicated by every available means.” (p 326 “Here I Stand” by
Roland Bainton, Lion 1988)
He would not be speaking of this Spirit-filled Church that abides by the holy word of God.
However, Luther wrote huge numbers of tracts and delivered huge numbers of sermons. Even some of his private conversations have been written down by others, at the time, so that finding nine words he is purported to have said (or written) is truly to search for a needle in a haystack. I have not come across it in the book I quote from.
But this might be worth pointing out from p. 306 (Ibid):
"We here are of the conviction that the papacy is the seat of the true
and real Antichrist...personally I declare that I owe the Pope no
other obedience than that to Antichrist." (Aug. 18, 1520) Taken from
The Prophetic Faith of Our Fathers, Vol. 2
In Luther's day, a theme of contrasting Christ with the pope was exploited in many pamphlets printed by the Protestant press. Luther was involved in a lot of them though he was helped by a vast cohort of others. In a cartoon entitled "Christ Disarms the Pope", the pope is on a war-horse accompanied by the devil; Jesus is on a donkey carrying a cross. Such cartoons were reciprocated. The Catholics produced a cartoon of Luther in league with Lucifer, along with many others.
In one Protestant skit, Christ is made to say, "I have not where to lay my head." The pope comments, "Sicily is mine. Corsica is mine. Assisi is mine. Perugia is mine."
In another Christ says, "He who believes and is baptized will be saved." The Pope says, "He who contributes and receives indulgences will be absolved."
Christ - "Put up your sword." The pope - "Pope Julius killed sixteen hundred in one day."
Christ - "Feed my sheep". The pope - "I shear mine."
There was no love lost between Luther and the papacy, as those cartoons clearly showed.
If no source for your quotation is provided, I think you may safely assume that, if Luther said or wrote those nine words, they were aimed at the Church territorial, which would include the pre-reformation church as well as the one powerfully active in his day. If a better answer is provided and I am wrong, I will acknowledge that.
EDIT - in view of Korosia's answer, I acknowledge that Luther could, indeed, have been referring to the 'true church, as here where you quote:
Luther notoriously called the church 'the greater sinner'. He said that, just as there is no-one who is a greater sinner than the Christian, so "there is no greater sinner than the Christian church", and that is why, he added, the chirch prays daily "Forgive us our trespasses". Changing the Church: Transformations of Christian Belief, Practive, and Life; Mark D. Chapman, Vladimir Latinovic; Pg. 245
A point that might be relevant is that sin in unholy things is bad enough, but if those in the Church sin in holy things, that is far, far worse as Christians are to be holy, with pure worship. If Luther had that in mind - the reformed church sinning in holy matters, that could account for such a claimed statement.