Faith in Christ comes first, which is a miracle of the Holy Spirit in the spirit of man.
“Flesh gives birth to flesh, but the Spirit gives birth to spirit” John 3:6
“The spirit of man is the lamp of the Lord, searching all his innermost parts.” Proverbs 20:27
The spirit of man (like a lamp requires oil) requires the Holy Spirit to function as God intended. At the Fall of Man the Holy Spirit departed from Adam and Eve’s spirits. Jesus made it possible for our Heavenly Father to send the Holy Spirit to once again indwell believers spirits.
Without saving faith, you only have human reason upon which to base your decision concerning the truthfulness of Christianity. You need to be raised up to a higher level in order to know for sure that Christianity is true.
Would you like to receive the forgiveness of sins as well as sanctified reason from your Creator?
If so, “Repent and believe the good news of what Jesus has accomplished for you” (Mark 1:15). In those few words, Jesus presented man with the only path to eternal life on a new earth and heaven. Are you on it? And if not, do you want to be on it?
The Apostle John under the inspiration of the Holy Spirit taught people how to know if their religion was fake:
“Whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him, but whoever keeps His word, in him truly the love of God is perfected. By this we may know that we are in Him: whoever says he abides in Him ought to walk in the same way in which He walked.” 1 John 2:4-9
“By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.” 1 John 3:10
“We know that we have passed out of death into life, because we love the brothers. Whoever does not love abides in death.” 1 John 3:14
The Apostle John also taught people to know if they had eternal life.
“I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life” 1 John 5:13
This knowledge of eternal life through faith in Christ is what then confirms to a believer that Christianity is true.
“The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.” Romans 8:16
This explains how I know Christianity is true. What about you? Are you trusting Christ alone for salvation? It is the only way to be forgiven of your sins, and the only way to receive the anointing and sanctified reason. This is how you can come to know that Christianity is true.
Without the new birth by the Holy Spirit, a person remains in spiritual darkness and only rises as high as his intellect and human reason can take him.