I wish there was a simple answer. From man's point of view, it is that he has free will to accept or reject the gospel. No one knows who will be saved and who will not except God Himself. God's word has tremendous power. Through the work of the Holy Spirit and with the Word of God, which is the sword of the Spirit, sinners can be convicted of their sin and also come to know that Lord Jesus alone can save them.
God chooses to save through His Word. He created the universe through His Word. Obviously, God's Word is a reflection of who God is. Christians sure need a better revelation of the power of God's Word. We need to remember that Lord Jesus did not just preach God's Word, He IS God's Word!
From God's point of view, we are foreknown, predestined, called, justified and glorified (Romans 8:30). Are we predestined because God knew that we would accept Christ before we were even born? I don't know. I dare not say that there is no need to witness because God is going to save people anyway.
I think we need to keep it simple. Farms were different in Jesus' time. They were not the huge, fenced off lands that are common today. There were paths through the fields and no bulldozers to clear away rocky areas. The sower's job is to sow. The seed has life within it but it needs the right conditions to sprout and become deeply rooted. The sower is not responsible for the state of the soil.
Often, people hear the gospel a number of times before the "seed" takes root. The other need for growth is water. So we need also to preach the Word even if there seems to be no response. We don't know if someone else will water the seed that we've sown. I spent hours witnessing to my sister, many years ago. (She lives in New Zealand) She seemed unmoved. Yet she said later that the word burned within her. A year later, she went to a meeting where she accepted Christ.
When it comes to getting along, we need to know that it's the unity of the Spirit that matters. Unity of doctrine is not the ultimate and is probably unachievable in this life! We can and should agree to disagree on matters that are not central to salvation.