Jesus says that if we pray within God’s will and in His name, He will grant our prayers. The Bible also states that it’s God’s desire that everyone would come to Christ. Is that an assurance that if we pray for someone’s salvation, they will be saved?
It’s true that God doesn’t want anyone to perish but for all to come to repentance (2 Peter 3:9). And it pleases God when we pray for others because He wants everyone to be saved and come to a knowledge of the truth (1 Tim. 2:3-4). But we have to remember that salvation is a personal choice so not even He can force someone to accept Him.
However, our prayers will cause the Holy Spirit to exert a strong influence on the people we pray for, making it very difficult for them to resist. And getting others to agree with us in prayer will make resistance even more difficult.
I believe God’s word tells us He will eventually win these battles for us. His word tells us the prayer of a righteous person is powerful and effective (James 5:16), and we have a righteousness from God that comes through faith in Jesus Christ to all who believe (Romans 3:22).
In short, there’s nothing we can do that will have a greater effect on changing another person’s eternal destiny than unrelenting prayer on their behalf.