Humans exist within the purview allowed by God. If He hardens a person's heart, they are not able to put their trust in Christ. So simple a child could understand. Our will is autonomous, but constrained by God's limiting allowances. We make plans, but God directs (limits) their implementation. And even our plans are limited to what God allows us to plan. God allows us to sin, and then holds us accountable for our choices. God allows us to put our trust fully in Christ, and can choose to credit our faith as righteousness. So our salvation does not depend on our coming to "saving faith" as if we earned or merited salvation, but on whether God chooses to credit our faith or not. Romans 9:16.