How can I deal with my father's unjustified rejection of my engagement to a young man I love and who is religious and of good character, especially since I have sought God's guidance through Istikhara?

A Muslim knows that this world is a house of trial, and patience in the face of calamities and a servant's contentment with Allah's decree elevate one's ranks and expiate sins. Allah has legislated that a woman should have a guardian (wali) to oversee her marriage, and this is out of Allah's mercy and to safeguard the interests of His servants. Therefore, it is advisable to seek help from trustworthy and sincere family members, and to perform Istikhara (prayer for guidance) to Allah, the Almighty, for Allah only decrees what is good for the believer. You can try to persuade your father regarding the person you wish to marry, but with a calm dialogue, avoiding insistence and argumentation, and earnestly supplicating to Allah with sincerity. Finally, maintain piety towards Allah, for a calamity in one's religion is the greatest of calamities.