Is a man entitled to back out of his promise to marry a woman to whom he was previously attached, after his religiosity increased and he found that she is not suitable for him Islamically, knowing that another man had proposed to her then withdrew?

The young man should praise God for His grace and guidance. He is not obligated to marry the girl he promised. It is permissible to break off the engagement if there is a valid reason, such as her unsuitability, especially if the engagement has not yet been formalized. He should seek a woman of good religion and character, and sever his relationship with this girl and with any other woman who is not a mahram to him.