What is the ruling on marrying a girl who believes in the existence of God and the three monotheistic religions, but does not follow any religion, and has the potential to learn about Islam, but it is not certain that she will embrace it?

The Sharia has permitted a Muslim man to marry a Muslim woman or a chaste woman of the People of the Book. He is not permitted to marry any others. A woman's mere belief in the existence of the Creator does not legitimize marriage to her. If you desire her conversion to Islam, invite someone who can explain the truth to her. If she embraces Islam, then marry her; otherwise, seek out a righteous Muslim woman. Beware of falling into an unlawful relationship with her.