The answer can be found in Ephesians 4:11-16, where the Apostle Paul wrote: "And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers". Now let me just say at the outset that I do not believe there are any more apostles or prophets or evangelists in the church. These men were given to the church in times in which we did not possess the complete revelation of God. But now, we do have the Word of God all that will be revealed by God us from God. We have the Word of God. 2 Peter 1:20-21, 1 Thessalonians 2:13
God has given pastors and teachers to the church, and He has gifted these men to be under-shepherds to the "ekklesia" or to the church of Jesus Christ, to those that have been "called out of the world and from their sins unto Christ". In Christ, Christian, God has called YOU Christian to be under pastors and teachers. What is the pastor's primary calling to the church? Pastor literally means to shepherd and oversee the flock. 1 Peter 5:2: "Feed the flock of God" saith Peter to the elders of the NT church, feed them with the truth of God's Word! So, (Jeremiah 3:15: "And I will give you pastors according to mine heart, which shall feed you with knowledge and wisdom"). So why do you need a pastor?
First: Paul wrote: "For the perfecting of the saints": They are given to YOU Christian so that you would grow in grace and in the knowledge of our Lord and Saviour Jesus Christ. This word "perfecting" means that you might "mature and grow up" spiritually in the Lord.
Second: for the "edifying of the body of Christ": Edify means to build up; understand Christian, you will not be built up spiritually without a pastor! You might think that you can read and study the Word of God on your own, and you should, but don't think to yourself that you will grow and become mature in Christ, or be built up upon your most holy faith unless you are obedient to what God has appointed for you in your Christian life, (a church and a pastor).
And then, Ephesians 4:13: "Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto the perfect man, unto the measure of the stature of the fulness of Christ". Are you, Christian, ready to boast that you have arrived (unto the measure of the stature of the fulness of Christ) and that you need no more instruction from the Word of God? I don't believe any spiritually-minded Christian would think or say these things. This is why you need a church and a pastor!
Third: God has given pastors to the church; "that we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive"
Christian, you need pastors that are going to watch for your soul; pastors who are going to teach and preach to you the Word of God; who are going to love you enough to tell you when you are straying or when you beginning down the wrong path in your life. And......in those times when you are being tossed to and fro by those that would deceive you. Make no mistake about it, if you're a child of God, Satan is going to attack you in your mind and do his worst to turn you away from Christ and back to the world and back to your former life of sin.
If you profess to be a Christian, but you don't have a church; find one that preaches the truth of God's Word. Get yourself under a godly pastor, who will teach you and love you and watch for your soul!