He is calling himself a sinner because his doctrine determines him to be so.
Many Christians will also call themselves sinners. Likewise, other believers know themselves to be saints. What's important though is what the bible says about them that believe.
In most of Paul's letters he calls the people in the church (the believers) "saints". He even addresses the Corinthian church as a people who are called to be saints. Remember, this was one messed up church - there were men who were having sexual relations with their father's wife among other perversions. Yet their calling was to "be" saints.
Think about that. Do you think that men who had had this horrid stuff going on in their lives qualify to be saints? According to scripture - YES.
The very same that Jesus called to salvation.
Unto the church of God which is at Corinth, to them that are
sanctified in Christ Jesus, called to be saints, with all that in
every place call upon the name of Jesus Christ our Lord, both theirs
Here's another verse that reveals saints as not perfect people but people who are being perfected. That's a huge difference. Saints can sin, but it's not right behaviour.
For the perfecting of the saints, for the work of the ministry, for
the edifying of the body of Christ: - Eph 4:12
The word saint means in greek - most holy thing. Counter intuitively the word Holy also means the same thing, Saint or Holy. So, what does it really mean to be Holy or to be a Saint?
Most Christians think that to be Holy means to to good, to keep God's laws. But:
All have sinned and fallen short of the glory of God - Romans 3:23
So this is not an accurate definition of what Holiness is. And likewise, I haven't met a christian yet who hasn't sinned after becoming a believer.
Whatever it means to be Holy or a Saint must have nothing to do with us, because no one qualifies on their own goodness. Romans makes that pretty clear in the first couple of chapters. We have nothing to boast about and the only way to be saved is through faith in God's saviour Jesus.
You'll also notice that in the OT, certain things in the temple were Holy.
Q. What made them Holy? My definition of Holy is set apart. Likewise, believers (christians) are set apart. We are told to be in the world and not of the world. We are told to be ambassadors for Christ 2 Cor 5:20. We now represent Him. To do that, we need to know who were are in Christ.
Here's a glimpse which is a little too much to write in a few paragraphs but:
When we believe we are born again. We receive a brand knew spirit in our inner man (the new man) and we now have the mind of Christ. 1 Cor 2:16. The old man is dead (your old spirit) The Spirit and the Soul are not the same thing. Your spirit is the new creation. Your soul and body are yet to be renewed, but your spirit is perfect, it is holy.