This conflict is addressed all through Scripture. In fact, it was the issue even in the Fall of Adam and Eve. They were promised eternal life as the reward for obedience, and spiritual death for disobedience (Genesis 2:17). The test for their obedience was one thing: the ban on eating from the Tree of the Knowledge of Good and Evil. When Satan came to tempt them, this was also the point where he applied his best temptation (Gen. 3:5): "God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil." The test of the tree was not about a mere piece of fruit. Rather, the test was over Adam's source of authority. Would it be God? Or would it be himself? This was also the focus of Satan's attack: "Will you allow God to determine everything for you, Adam?" That is, would authority be monergistic? "Or will you be like God, Adam?" That is, would it be synergistic? And we know Adam's choice. We also know the consequence upon his posterity: "Therefore, sin came into the world through one man, and death through sin, and so death spread to all men because all sinned. One trespass led to condemnation for all men" (Romans 5:12, 19). While this is most visibly a reference to physical death, its real significance is to the death of the human spirit (Ephesians 2:1): "You were dead in the trespasses and sins." God created a monergistic plan for eternal life. However, Adam and Eve chose a synergistic plan, and, instead, lost that very life. That is, synergistic salvation is really a plan for eternal death, not life.
We must be thankful, however, that monergism didn't cease merely because Adam rejected it. Rather, the same God determined, without any input from fallen men, that He would monergisticly redeem men. The same prophet, Moses, reports this in Deuteronomy 30:6: "The Lord your God will circumcise your heart and the heart of your offspring, so that you will love the Lord your God with all your heart and with all your soul, that you may live." Notice that He doesn't offer a new heart. He gives one. He doesn't request that we love Him. He determines that we shall. This is repeated in the prophets (Ezekiel 36:26-27): "I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put My Spirit within you, and cause you to walk in My statutes and be careful to obey My rules."
We see so clearly in both verses that God doesn't merely offer salvation. That would be a synergistic, or semi-Pelagian plan. Rather, He completely saves those whom He has chosen. That is monergism.
Jesus saves His people from our sins (Matthew 1:21). He is not merely a cheerleader on the sideline hoping that we might be saved.