Is it permissible to invite people to the right path by frightening them with the invalidity of marriage due to abandoning prayer, or contracting cancer due to plucking eyebrows, and will their guidance based on such fear, and not on the fear of Allah, be accepted?

Allah Almighty gives glad tidings to those who obey Him and warns those who disobey Him. The means of inviting to Allah should be through encouragement (targhib) and deterrence (tarhib), both regulated by Islamic law. The preacher should mention verses and hadiths of warning, Allah's power and His grip on the disobedient, and that disobedience is a cause of afflictions and evils, while obedience is a cause of honor and happiness. There is no harm in mentioning the differences of opinion among scholars on issues such as the one who abandons prayer, while trying to win over the heart of the admonished and clarify the seriousness of his sin. As for lying and mentioning things that have no basis to frighten people, it has serious negative consequences, as the admonished may lose trust in the preacher. Moreover, it is speaking about the religion of Allah without knowledge. Rather, religion is a trust, and one should only speak about it with knowledge.