What many unbelievers might not understand or are not aware of is that in addition to the promises of the Bible, believers have the assurance of their relationship with Christ because the Holy Spirit testifies with their spirit that they are children of God. They sense the presence as well as voice and leading of God’s spirit in their lives. Sometimes the Holy Spirit even manifests his presence in the lives of believers.
The Bible says the Spirit of God dwells in God’s people. Anyone who doesn’t have the Spirit of Christ doesn’t belong to him. (Romans 8:9). When a person takes the step of faith to believe in Christ and gives his heart to him, confessing and repenting of their sins, he forgives and saves them. At that very instant the Holy Spirit simultaneously indwells that person. Christians know for certain God is real and the Bible is the Word of God. They know they’ve been saved and they know Christ personally. They know these things because the Holy Spirit is inside of them and he testifies with their spirit the reality of Christ.
Because the Bible in its original writing is the inspired, inerrant, God-breathed Word of God and because the Holy Spirit lives in believers and opens up their hearts and minds to the Holy Scriptures, the Bible comes alive to them as they read it. Unlike any other book ever written, it’s a living, active book that speaks to our soul and spirit. (Hebrews 4:12). God’s Holy Spirit in believers also helps them in spiritually discerning its message. (1 Corinthians 2:14).
The Bible not only tells us we’re indwelt by the Holy Spirit at the moment of salvation, but also at that moment we become a part of the body of Christ, the fellowship of all believers, as the Holy Spirit baptizes us into that body. (1 Corinthians 12:13).