How can I know that what I am believing is the truth?

Answer:

You know the truth by going to the source (the Bible) and study it for yourself. That is why I try to cite references for my answers. I don't want people to just take my word for it. I want them to see what God said for themselves. "If anyone speaks, let him speak as the oracles of God. If anyone ministers, let him do it as with the ability which God supplies, that in all things God may be glorified through Jesus Christ, to whom belong the glory and the dominion forever and ever. Amen" (I Peter 4:11).

Peter said, "And so we have the prophetic word confirmed, which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts; knowing this first, that no prophecy of Scripture is of any private interpretation,  for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit" (II Peter 1:19-21). Men don't get to pull out passages and say "This is what God meant to say." That doesn't stop people from trying, but it does tell us that God's word can be read directly.

In the case of Lucifer, God was talking to the king of Babylon (Isaiah 1:4). He used various illustrations to show the king's faults, one of which was to draw a parallel between him and someone named "Morning Star" or Lucifer. "How you are fallen from heaven, O Lucifer, son of the morning! How you are cut down to the ground, You who weakened the nations! For you have said in your heart: 'I will ascend into heaven, I will exalt my throne above the stars of God; I will also sit on the mount of the congregation On the farthest sides of the north; I will ascend above the heights of the clouds, I will be like the Most High.' Yet you shall be brought down to Sheol, To the lowest depths of the Pit" (Isaiah 14:12-15). It is about Lucifer and yet it is about the king too who acts like Lucifer -- both at the same time. Lucifer fell from heaven when he sought to place his throne above God. He sought to make himself equal to God. In punishment, he was cast down to the abyss.

Thus, this is hyperbole. We see a king who thought himself so mighty that he could challenge God Himself.  Yet there are terms here that go beyond what we would ascribe to a mere man, which is why we call it hyperbole.  Men hope to reach heaven when they die. Lucifer was cast down from heaven.  Men may dream of power, but few claim to place their throne in heaven and challenge the rule of God.

Hyperbole doesn't mean the illustration is a complete fantasy. It can be based on something else that normally would not apply to a man. In this case, we know that Satan was cast from heaven. "And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought, but they did not prevail, nor was a place found for them in heaven any longer. So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him. Then I heard a loud voice saying in heaven, "Now salvation, and strength, and the kingdom of our God, and the power of His Christ have come, for the accuser of our brethren, who accused them before our God day and night, has been cast down" (Revelation 12:7-10). We know that Satan was cast into the Abyss. "And the angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day" (Jude 6). Not keeping their proper domain tells us that these angels tried to reach for what was not theirs to have.

They are not one and the same but one behaving like the other.