The writings of Luke the physician starting with his version of the gospel - Luke 13:1-5 comments: cause and effect

Luke 13:1 ¶  There were present at that season some that told him of the Galilaeans, whose blood Pilate had mingled with their sacrifices. 2  And Jesus answering said unto them, Suppose ye that these Galilaeans were sinners above all the Galilaeans, because they suffered such things? 3  I tell you, Nay: but, except ye repent, ye shall all likewise perish. 4  Or those eighteen, upon whom the tower in Siloam fell, and slew them, think ye that they were sinners above all men that dwelt in Jerusalem? 5  I tell you, Nay: but, except ye repent, ye shall all likewise perish.

Jesus continues, as someone speaks of Pontius Pilate, the Roman governor, and his atrocities against Galileans at worship. He responds with a question that points at reality. When someone suffers do you think they were being singled out as the worst of the worst? Are there not many others just as evil or sinful who go unpunished? But, if you do not repent and turn from your sins and turn towards God you will also perish.

He brings up a point about an accident, the falling of a tower that killed eighteen persons. Again, He asks them if they think these victims were the worst people in Jerusalem. Of course not, but unless you repent you, too, shall perish.

Repenting of your sins against God and turning to Him is a fundamental doctrine of Christ. Paul wrote;

Hebrews 6:1 ¶  Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,

Dead works are defined Biblically as works of the flesh, sins. Read Hebrews, chapter 9 for the context.

Hebrews 9:14  How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?

To repent is to turn from something, changing your mind about it and rejecting it.

Exodus 32:12  Wherefore should the Egyptians speak, and say, For mischief did he bring them out, to slay them in the mountains, and to consume them from the face of the earth? Turn from thy fierce wrath, and repent of this evil against thy people.

Ezekiel 18:30  Therefore I will judge you, O house of Israel, every one according to his ways, saith the Lord GOD. Repent, and turn yourselves from all your transgressions; so iniquity shall not be your ruin.