It is true that after Luther and Erasmus had their falling out, from an initial weak respect for one another, Luther made several comments that imply he considered Erasmus a man without genuine faith.
Most of what he said is recorded by others during conversations which has been compiled in the 'Table Talk' volume of Luther's works. Prior to this information there is a letter to Erasmus directly when they were on better terms and then of course their open dispute about predestination and free will explained here.
To answer the question of whether Luther considered him an unbeliever because he was Catholic, or for some other reason, I would say that Luther considered most Catholics unbelievers, but only if they rejected his teaching about the doctrine of justification by faith. As Erasmus seemed to not believe in this fundamental article of what Luther saw as the most essential part of true Christianity, yes, he assumed Erasmus was like many Catholics, and was not a Christian as he had no genuine faith.
In the table talk volume there is around a dozen mentions of Erasmus, and actually Luther speaks much more harshly about many, many, others. So these comments are actually mild according to Luther's style and wide scope of declaring the guilt of many in his day.
I think the most revealing comments are these two (from Luther's Table Talk):
No. 4028: An Appraisal of the Deceased Erasmus September 30, 1538
“Erasmus of Rotterdam wrote many excellent things because he had talent and leisure, was without worries and official duties, didn’t preach or lecture, and was no businessman. In his manner of life he was without God, lived with a sense of great security, and died the same way. When he was in the agony of death he didn’t ask for a minister of the Word or for the sacraments. It’s a fabrication that in the agony of death he may have spoken these words of confession, ‘O Son of God, have mercy on me!’ God forbid that in my last hour I shouldn’t want to have a godly minister of the Word, that I couldn’t summon the nearest one at hand, that I shouldn’t want to thank God! That fellow Erasmus learned such things in Rome. But now one ought not talk about this on account of his reputation and his books.”
This being the second:
No. 3033b: Prefaces to the New Testament by Erasmus April 1, 1533
On April 1 Martin Luther spent almost the whole day reading Erasmus’ prefaces to the New Testament. He said excitedly, “Even if this snake is so slippery that he can’t be caught, we’ll nevertheless condemn him with his writings, we and our church. Although many wise people in the church may be offended, yet it’s better that we lose them than deny Christ, the Savior. I’ve had more experience with this Savior than I can ever say and thank him for.
“Erasmus has indeed put out very disgraceful prefaces, even if he did soften them, because he makes no distinction between Christ and Solon. Moreover, as the prefaces to Romans and to the canonical First Epistle of John show, he obscures the authority of Paul and John, as if these writings were of no moment, as if the Epistle to the Romans had no relevance for our time, as if the difficulty in this epistle outweighed its value, etc. Is this praise for the book’s author? For shame