Jesus said that anyone wanting to be His disciple would be required to deny themselves, take up their cross, and follow Him.
In Jesus' day, there were crowds who walked alongside Him and His disciples, but they weren't really following Him. They consumed what Jesus said but did not heed His words. They had given up nothing and continued to live the lives they had. In other words, they did not deny themselves. Sadly, this still goes on today as many believe they just need to "show up" in church; these people have not allowed Jesus to be the leader of their lives.
Jesus was well aware of this and goes on to ask us this: What point is it to gain the whole world if we lose our soul?
Often one who is living the ways of the world may be so comfortable in them that they may not even realize that there is a better path to follow - the Way that Jesus taught.
As a Jewish man working to collect taxes for the Roman Empire, Matthew "had it all," yet he was considered an outcast. And sadly, he had grown accustomed to that, for he simply did not know any other way.
Matthew held an important position in his community; yet he was hated by nearly everyone - especially his own people. Matthew had the protection of his Roman employers; yet he was fearful. Matthew had a lucrative career that made him wealthy and powerful; yet he was unfulfilled.
Then Jesus called him, and his eyes were opened! Matthew immediately chose to follow Him and in doing so, he went all in. He walked away from his position, his protection, his power, and his wealth.
When we enter into a relationship with Christ, we become a new creation and our old self passes away. The decision to follow Jesus is a bold decision, not to be taken lightly. It is a decision that will change one's life - at the cost of their worldly ways. Those changed lives will be clearly evident to those around them. Some will even wonder if they have lost their mind (imagine the reaction of the Roman authorities when Matthew turned in his keys to the tax booth!). And yet such a decision is the most important decision we will ever make - the only one that will give us eternal life.
Read: Mark 8:34-36, Matthew 9:9-12, Matthew 10:2-3, 2nd Corinthians 5:16-17