Is it true that Martin Luther of the Reformation said “Be a sinner, and let your sins be strong.” Yes, it is. But the second half of that sentence was “but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world.”
Here is that passage in it’s original context; a letter from Luther to Melanchthon on August 1, 1521. If you like you can scroll directly to number paragraph thirteen for the complete passage.
This is also the letter where Martin Luther expresses his favor for allowing monks to marry.