The words God and devil are elaborately described in various religions. Faith is required to believe in all the descriptions. However, the existence of God and the devil can be plausibly explained. The word God (a Supernatural Being) is from the Western Civilization. Other civilizations have different words for the same concept which is Ultimate Reality. Faith is not required to believe in the existence of Ultimate Reality. Simple logic in accordance with generally accepted science is sufficient: only force applied by an energy source does work. Also, modern technology has revealed that an energy source can apply force remotely in order to do work that is some distance away from the energy source. So, to the extent that the creation of the Universe is Ultimate Work, one must accept the existence of Ultimate Energy beyond which nothing else is known. Hence, Ultimate Energy is Ultimate Reality (the concept to which the word God has been assigned).
The human perspective of the Universe is able to reasonably establish Ultimate Reality as an Ultimate Energy Source. Humans who believe in a Super Natural Being have assigned the name God to this Ultimate Energy Source. There is nothing implausible about such naming. Also, these believers believe that the creation of the Universe in general is Good. Hence, they believe that Ultimate Reality Who did the Work (the Creation of the Universe) is a Good Force. The establishment of good established evil (the antinome of good). In other words, a spectrum of not good exists because good exists. Humans call the extremes of the spectrum, evil. By Newton's First Law Of Motion (an object at rest will remain at rest and an object in motion will remain in motion at uniform velocity unless acted upon by an unbalanced force) good and evil are products of good force and bad force respectively. When the resultant force (unbalanced force) is the good force, good is produced. When the resultant force is the bad force, evil is produced. Hence, humans called the bad force the devil.
Both the good force and the bad force exist in the spirits of all humans. From the spiritual realm they manifest their work in the physical realm. The devil thrives whenever there are myriads of evils in the physical realm. Jesus Christ revealed some of the evils purveyed by the devil when he described the devil as a perpetual murderer and liar (John 8:44 KJV). St Peter described the devil as a roaring lion, walketh about, seeking whom to devour (I Peter 5:8). St James informs us on how to stop the devil from thriving: Submit yourselves therefore to God. Resist the devil, and he will flee from you (James 4:7). In other words, humans inability to resist the devil due to weak spirits devoid of God, is the reason why the devil thrives.
A spirit that submits itself to God eventually attains the Spiritual Angle. At which time the devil flees.