JC Ryle’s little book – A Call To Prayer – is a great and easy little read. Here he is on private prayer as a mark of genuine conversion:
This is one of the common marks of all the elect of God, “They cry unto him day and night.” Luke 18:1. The Holy Spirit who makes them new creatures, works in them a feeling of adoption, and makes the cry, “Abba, Father.” Romans 8:15. The Lord Jesus, when he quickens them, gives them a voice and a tongue, and says to them, “Be dumb no more.” God has no dumb children. It is as much a part of their new nature to pray, as it is of a child to cry. They see their need of mercy and grace. They feel their emptiness and weakness. They cannot do other wise than they do. They must pray.
I have looked careful over the lives of God’s saints in the Bible. I cannot find one whose history much is told us, from Genesis to Revelation, who was not a person of prayer. I find it mentioned as a characteristic of the godly, that “they call on the Father,” that “they call upon the name of the Lord Jesus Christ.” I find it recorded as a characteristic of the wicked, that “they call not upon the Lord.” 1 Peter 1:17; 1 Corinthians 1:2; Psalm 14:4.
I have read the lives of many eminent Christians who have been on earth since the Bible days. Some of them, I see, were rich, and some poor. Some were learned, and some were unlearned. Some of them were Episcopalians, and some were Christians of other names. Some were Calvinists, and some were Arminians. Some have loved to use liturgy, and some to use none. But one thing, I see, they all had in common. They have all been people of prayer.
I have studied reports of missionary societies in our own times. I see with joy that lost men and women are receiving the gospel in various parts of the globe. There are conversions in Africa, in New Zealand, in India, in China. The people converted are naturally unlike one another in every respect. But one striking thing I observe at all the missionary stations: the converted people always pray.
I do not deny that a person may pray without heart and without sincerity. I do not for a moment pretend to say that the mere fact of a people’ praying proves everything about their soul. As in every other part of religion, so also in this, there may be deception and hypocrisy.
But this I do say, that not praying is a clear proof that a person is not yet a true Christian. They cannot really feel their sins. They cannot love God. They cannot feel themselves a debtor to Christ. They cannot long after holiness. They cannot desire heaven. They have yet to be born again. They have yet to be made a new creature. They may boast confidently of election, grace, faith, hope and knowledge, and deceive ignorant people. But you may rest assured it is all vain talk if they do not pray.
And I say furthermore, that of all the evidences of the real work of the Spirit, a habit of hearty private prayer is one of the most satisfactory that can be named. A person may preach from false motives. A person may write books and ,make fine speeches and seem diligent in good works, and yet be a Judas Iscariot. But a person seldom goes into their closet and pours out their soul before God in secret, unless they are in earnest. The Lord himself has set his stamp on prayer as the best proof of conversion. When he sent Ananias to Saul in Damascus, he gave him no other evidence of his change of heart than this, “Behold he prays.” Acts 9:11.
I know that much may go on in a person’s mind before they are brought to pray. They may have many convictions, desires, wishes, feelings, intentions, resolutions, hopes, and fears. But all these things are very uncertain evidences. They are to be found in ungodly people, and often come to nothing. In many a case they are not more lasting than the morning cloud, and dew that passes away. A real hearty prayer, moving from a broken and contrite spirit, is worth all these things put together.
I know that the Holy Spirit, who calls sinners from their evil ways, does in many instances lead them by very slow degrees to acquaintance with Christ. But the eye of man can only judge by what it sees. I can not call anyone justified until they believe. I dare not say that anyone believes until they pray. I cannot understand a dumb faith. The first act of faith will be to speak to God. Faith is to the soul what life is to the body. Prayer is to faith what breath is to the body. How a person can live and not breathe is past my comprehension, and how a person can believe and not pray is past my comprehension too.A Call To Prayer, II. I ASK AGAIN WHETHER YOU PRAY, BECAUSE A HABIT OF PRAYER IS ONE OF THE SUREST MARKS OF A TRUE CHRISTIAN, https://www.gracegems.org/SERMONS/call_to_prayer.htm