Even if God has already determined the end of all things, we act in faithful obedience because God uses means.
One of the clearest places this is seen is in Acts 27. The Apostle Paul is on his way to Rome to stand trial before Caesar, when his ship is tossed by a furious storm. An angel promised Paul that he and the entire crew would arrive safely in Rome (Acts 27:23-26). They would be shipwrecked and storm-tossed, but God guaranteed they would survive. Nevertheless, Paul did not view God's promise as a reason to kick back and relax. He viewed it as a call to action.
Acts 27:33-38—As day was about to dawn, Paul urged them all to take some food, saying, “Today is the fourteenth day that you have continued in suspense and without food, having taken nothing. (34) Therefore I urge you to take some food. For it will give you strength, for not a hair is to perish from the head of any of you.” (35) And when he had said these things, he took bread, and giving thanks to God in the presence of all he broke it and began to eat. (36) Then they all were encouraged and ate some food themselves. (37) (We were in all 276 persons in the ship.) (38) And when they had eaten enough, they lightened the ship, throwing out the wheat into the sea.
If God has promised that you and all the sailors will survive and make it to Rome, why eat food? In chapter 28, after the entire ship has run aground on the island of Malta, Paul is seen gathering wood for a fire to warm his shipmates. Why bother fighting off hypothermia with a warm fire? Because God ordains the ends and uses means.
I think verse 34 may be one of the most helpful verses in Scripture to understand the interchange between divine sovereignty and human responsibility. Notice Paul urges them to take food. He doesn’t suggest it, he urges it. He’s seeking to persuade them. It matters to him that they listen. He wants them to exercise human responsibility. Why? Because they’ll die if they don’t? Well that’s true, but it’s not what Paul says. He tells them at the end of the verse that not a hair on their heads is going to perish. There’s divine sovereignty. God will keep His promise. God has already ordained how this story will end and nothing is going to stop Him from accomplishing all that He wills.
So why eat? Because it will give them strength. Here’s the deal, Christian. We act faithfully in life, not because God needs our help accomplishing His purposes, but because our experience in this life is greatly enhanced when we’re faithful. Your faithfulness to care for your physical body will not add to the days God has numbered for you, but it may give you strength along the way. Your faithfulness to share the Gospel with your unbelieving neighbor will give you joy in the journey. Your faithfulness to rightly discipline and instruct your children will enhance your experience of parenting. Your diligence to resist sexual temptation will further your joy. We’re faithful because it pleases our Lord, it serves our neighbor, and it serves our own joy.
The doctrines of divine sovereignty and human responsibility form an anchor for the storm-tossed soul. God ordains the ends, and He uses means.