"Isn't it enough to believe in Jesus? Why do I need to be a part of a church?"
"The local church is the visible, tangible expression of Jesus."
God in His wisdom chose to build the spiritual house we call the church. He could have just laid the foundation, Jesus Christ, and left it there. But He decided that on Jesus Christ He would establish a spiritual building so that the world might be attracted to the foundation.
Believers are to be connected. The local church is the visible, tangible expression of Jesus. Charles Spurgeon described believers who are not connected to the church as "good-for-nothing bricks." What is a brick made for? What is a stone made for? To be part of a building (see 1 Peter 2:4-10). If you are a rolling stone Christian, you are not fulfilling God's purpose in your life.
When the church is made up of people who are living out the Gospel, living as a holy, loving, and caring community, only then will non-believers come and fall on their faces and turn to the Lord. When non-believers encounter the community of believers, living stones that are built on the Living Stone, when they see members of the church loving, forgiving, caring for, and serving one another, they will be awestruck with the power of God in the midst of His people. You want to be a part of that.