Because I teach and write about spirituality, occasionally I'm asked to comment on scientific studies about the efficacy of prayer. The research always seems to include the assumption that one person's prayers are essentially as acceptable as another's. One of the flaws with such studies is that they do not associate prayer with the gospel. No one can begin to understand prayer until they grasp what the gospel teaches us about prayer.
Through the gospel we begin to pray
The Bible, rather than assuring everyone that God hears their prayers, slams Heaven's door against all who think God will hear them despite their sins: "But your iniquities have made a separation between you and your God, and your sins have hidden his face from you so that he does not hear" (Isaiah 59:2). In one sense, of course, God hears everything. But in this text we're told that God does not hear with a view to answering those who sin against Him. And of course, since every person except Jesus has sinned against God, the hopes are dashed of everyone who thinks all it takes for God to hear is for them to pray.
In fact, the Bible is even more shockingly counter-intuitive in Proverbs 15:8: "The sacrifice [including prayer] of the wicked is an abomination to the Lord." Many people seem to think, "It's true, I'm not a dedicated Christian. But if I get into a difficult situation and humble myself to pray, and I'm really sincere, surely God will accept my prayer." Or they believe, "If God is merciful and loving He will look favorably on the prayers of those who come to Him when they're in real need and pray hard enough."
But this text tells us that, instead of being impressed, the Lord actually abominates these prayers. Why? Because such people believe God should hear their prayers based on their own temporary humility and piety. In other words, they believe their own righteousness - in this case expressed in a short-lived acknowledgement that they need God's help - obligates God to answer.
Instead of honoring God with momentary sincerity, these prayers insult Him, for they imply the work of Jesus wasn't necessary. It's as though they're saying, "The life and death of Your Son weren't needed in my case. It was all a big mistake. I believe you should hear me based on what I have done - especially in these prayers. I don't need what Jesus did in order for you to hear me." Could anything be more offensive to God?
When it comes to knowing and being heard by God, Jesus was unequivocal: "I am the way, and the truth, and the life. No one comes to the Father except through me" (John 14:6). Confidence that God hears our prayers cannot come from our sincerity, humility, or need. Rather, "we have confidence to enter the holy places [i.e. the presence of God] by the blood of Jesus" (Hebrews 10:19). Until a person comes in repentance to God through faith in the person and work of Jesus Christ - who alone can remove the sin that separates us from God - his or her prayers will not be answered.
Does God ever answer the prayer of a non-Christian? Many stories claim that He does. In reality, are these "answered prayers" simply God doing in His providence what He was going to do anyway? The clearest ground biblically is this: except for those prayers leading to salvation, we can give no assurance to anyone outside of Christ that God will answer their prayer. It is only through the gospel that we truly begin to pray. For only then - after Jesus has made us and our prayers acceptable to the Father - do the promises of prayer in the Bible apply to us.
Through the gospel we continue to pray
Once we respond to the gospel in repentance and faith and are adopted into God's family, our new relationship with our heavenly Father becomes markedly prayerful. No longer is prayer just an obligation or means to get what we want, for the gospel makes prayer a desire and not a mere duty. Through the gospel we receive the Spirit who causes us to cry out, "Abba! Father!" (Romans 8:15; Galatians 4:6) with new heavenward, Father-ward orientation. In other words, those who have received the Spirit through believing the gospel really want to pray. The Spirit of God compels us talk to God.
Moreover, after our transformation we pray by the Spirit to the Father in the name of Jesus (John 14:13-14). Instead of relying on our sincerity to turn the ear of God, we come in the righteousness of Christ. The gospel teaches us that God welcomes us "in the Beloved" (Ephesians 1:6), and so we always come to our Father in the name of His Son.
Prayer should still remain a discipline, for even with the God-given desire to pray it's easy to allow the crush of responsibilities to distract us from habits of prayer. But thanks to the grace of God in the gospel, our prayers are always welcome.