Is a person considered an infidel if he insists on consuming usury despite knowing its prohibition, and is his action considered a legitimization of sin, given that legitimization is the belief that it is permissible?

The "istihlal" (considering something unlawful as lawful) that renders its perpetrator a disbeliever is the belief in permissibility while knowing of its prohibition. As for persisting in a sin while acknowledging its prohibition, that is a grave sin, but it does not reach the level of disbelief, and its perpetrator is subject to the will of Allah.