How can one convince himself of the existence of Allah to avoid the torment of the Day of Judgment, and what should he do to make up for the missed Ramadan fasts due to heedlessness?

The existence of Allah is an innate matter, testified to by both scriptural and cosmic signs, and affirmed by sound reason. No one's faith is valid until they firmly believe in the existence of Allah, that He is the Creator and Sustainer of the universe, and the true God. Among what necessitates conviction is contemplating Allah’s magnificent creations, such as the sun, moon, stars, and planets, as well as the earth with its seas, rivers, living creatures, and human organs and senses. All of this points to the existence of Allah and the obligation of His monotheism.