I have often heard the following challenge leveled at those who have a high view of the sovereignty of God: "If God is sovereign, why does He need us to pray and evangelize? Why must I be involved if, in His sovereignty, He will accomplish his will and the elect will be saved?"
It is a fair question, one I have wrestled with in the past myself, but it betrays our tendency to think of God's commands in terms of His need rather than our own. For instance, when God gave us the ten commandments, it did not grow out of God's fear that the world would spiral out of His control if we didn't have some rules. Rather, God knows His creation so intimately that He knows that not committing adultery (and obeying all the other commandments) in the long run result in a happier, healthier humanity.
Perhaps you see where I am going with this now. Can God accomplish all He desires and ordains without my prayers to assist Him or "grease the works"? Of course. Can God save the lost without my participation in evangelism? Certainly. BUT . . . can I benefit from evangelism and prayer if I'm not actively participating in it? I think not, or at least not to the same extent.
You see, just as humanity benefits from living within moral guidelines God set for our good and ultimate pleasure, healthy Christians benefit from prayer and evangelism. When you become a Christian, you become a part of the body of Christ. Through this union, we are given the Holy Spirit and the heart of Christ. The "heart of flesh" that God puts in us desires the things of God. Nothing breaks that heart and makes it beat sluggish and slow like living a placid, menial Christian existence. God desires our obedience in prayer and the great commission for our own good.
Should we pray and evangelize because we love God and want to obey Him? Certainly this is the primary reason every Christian should. But I believe that our ultimate satisfaction and fulfillment as a child of God is at stake as well. Are evangelism and prayer hard work? Of course they are. But so is the fruit of the Spirit. Yet when I work at being more loving or patient or kind, I become a more satisfied and fulfilled Christian as those things come more naturally and with less work. So it is with prayer and evangelism. They should not be simply designated as tasks we do while the fruit of the Spirit (and the rest of the Christian walk) is seen as something to aspire to for happy, healthy Christian living.
Evangelism and prayer begin to make sense within God's sovereignty when we begin to see them in the context of the need of the Christian rather than that of God.