Obstinate doubt is voluntary doubt—doubt may be defined as either voluntary or involuntary. The Catechism of the Catholic Church explains,
Voluntary doubt about the faith disregards or refuses to hold as true what God has revealed and the Church proposes for belief. Involuntary doubt refers to hesitation in believing, difficulty in overcoming objections connected with the faith, or also anxiety aroused by its obscurity. If deliberately cultivated, doubt can lead to spiritual blindness. (CCC 2088)
Involuntary doubt is not, in itself, sinful and may be experienced by any sincere believer. Voluntary doubt, on the other hand, as a willful refusal to assent to God’s revelation, is a grave issue.