Doubts come from satan. He always tries to attack new christians saying they arent saved.
What we can do is encourage new believers that they have assurance they are saved. And that satan cannot pluck them out of Jesus hand. When Jesus saves, he saves completely.
We need to act on this knowledge.
The catholic religion teaches people that they do not have the assurance of salvation. Their doctrines, such as purgatory ensure that false belief.