Read: 1 Corinthians 16:13
The world can be a place filled with temptations of greed, pride, jealousy, hatred, etc. If you are not guarded, it is easy to fall into such kind of temptations. When suffering comes, it is easy to deny God and question your faith. Sometimes when people see innocents die or suffer, they stop believing in God. The devil brings in such doubts and confusion.