It's easy to understand.
If you want to know God, believe in Christ's death, burial, and resurrection for your sins. Period. No jumping through hoops or going through red tape to get to heaven--just faith.
But at the same time, the gospel is complicated. We tell people to believe in the Lord Jesus, while all the while knowing that they are incapable of following our advice. The spiritually dead don't respond to spiritual truths, in much the same way that our dead goldfish don't flush themselves down the toilet when we tell them to. In a way, every time we share the gospel, we are doing Zombie Evangelism. I can tell a corpse to get up and walk as many times as I want, but unless life reenters their body, something I'm not able give them, they will remain dead.
"And you were dead in the trespasses and sins in which you once walked...." (Ephesians 2:1)
But I think God designed it that way for many reasons. First, when we are powerless to make the dead come to life, He gets all the glory. After all, we try and try to convince people of their lostness and need for Jesus, but they remain stubbornly ignorant. So when God steps in after we have exhausted all our abilities, efforts, and strategies, we have to stand and say, "Lord, you are amazing."
Second, the more we grasp this concept--that we are powerless to convert people--the more we learn to depend on God in our efforts to share the gospel. Notice that I said 'powerless' and not 'helpless.' We are powerless. There is nothing we can do to make someone experience the new birth. But we are not helpless. In fact, the more we see we are powerless, the more we see how much help we have in God. The concept is: a continually diminishing sense of self-sufficiency that is replaced by an increasing awe at the all-sufficiency of God.
Third, as our consciousness of the first two ideas grow, we are freed to share the gospel with more boldness and to share it more frequently. As I realize that dead sinners are brought to life only by the soul resurrecting power of the creator of the universe, I become more confident in my witness. I mean, who is bigger than God. Or more intimidating. I'm the one with all the power behind me. Also, when I see my great need and dependence on God in evangelism, I am free to share it often. After all, the closer I come to God, the more I see how much He loves me, and how He humbled Himself even to the point of dying on the cross in order to give me life. If that is true, everything that matters is already mine and I've got nothing to lose. No amount of persecution or ridicule could ever undo the fact that Christ died to make me, counted among His enemies, on of His own.
Fourth, acknowledging God's sovereignty in evangelism frees us to be a lot less critical of other evangelists. Namely the ones who don't see the complexity of the gospel. This is where I am trying to grow. When Paul was in prison, there were a lot of people who were sharing the gospel just to spite him. Maybe they were sharing in a way that he didn't particularly like, so they took advantage of his imprisonment to prove to him that it could work. Maybe they wanted recognition, and since Paul was the biggest thing around in the church, they used his tribulations to gather their own following. But whatever the case, Paul was just glad they were sharing the gospel. Wow. I want to be like that someday.
So, we can take heart knowing that God uses us to share the gospel for His glory, and that He empowers us to do so by His strength. And because of this we are liberated to be bold and freely share it, and to be encouraged by those who would do it by different means than us.
And at least the people we're sharing with, though they may be the living dead, aren't trying to eat us.