The answers to this question vary from “pray for it” to “believe God’s promises”, and from “just wait” to “you have to accept it”. In his book Joy Unspeakable, Martyn Lloyd-Jones mentions three ways to reach assurance of faith.
The first step
The first stage comes from scripture. In a pastoral conversation with someone who is doubting his faith, it is helpful to remember the promise in John 3:16, “For God so loved the world, that He gave His only Son, that whoever believes in Him should not perish but have eternal life.” If somebody believes that the Bible is the Word of God and acknowledges the truth of this verse, he can draw the following conclusion: “If you believe the Bible, you should also believe this. The Bible says that you, because you believe, will not perish. You are saved, you must be saved. Do not rely on your feelings too much; believe the Word of God.” According to Lloyd-Jones, this is something that everyone should do. But he considers this to be only the first stage of certainty.
The second step
When people object to the first step by saying it is merely a mental acceptance of truth rather than real faith, Lloyd-Jones responds that the second step of assurance is based on a person’s actions. If you love your spiritual brothers and sisters and do not consider God’s laws a heavy burden, you may draw the conclusion that you have passed out of death into life (so 1 John 3:14).
The third step
In the first two stages, it is the person himself who draws conclusions. But in the third step, it is God Himself who gives full assurance. Romans 8:15-16 clarifies this, “Our spirit has cried out, Abba! Father!” Following this, the Spirit bears witness with your spirit. He confirms what you say. This is the immediate testimony of the Spirit and therefore absolute and certain. This can happen by a Bible verse or by an impression in your mind or heart. This causes inexpressible joy (1 Peter 1:8).
God calls on our willingness to believe
In some churches only the third stage of certainty is considered real. This leads to many people living in uncertainty of their salvation for a long time or even for their whole life because they have not had this special experience. This also results in becoming easy prey for Satan. But the Bible always calls on your will and readiness to believe. That is exactly what the Lord Jesus required from the father of the boy with the unclean spirit in Mark 9:14-29. This father had no choice but to confess his faith, as an act of faith. His feelings did not yet match his words, but they did not need to. All the rest was left to Jesus! Therefore as an example for you, he said, “I believe; help my unbelief!” (Mark 9:24)
The Savior considered this confidence in Him sufficient to heal the son. There was no special experience underlying this confession; it was an act of obedience. After his confession, the father witnessed a miracle, and his faith was strengthened by it. This is not about “free will,” but about accepting God’s call that comes to you through His word. Whoever answers this call can only do so by the working of the Holy Spirit.