This question is of special importance to Christians, because "if anyone does not have the Spirit of Christ, he is not His" (Romans 8:9). Jesus Christ promised that true Christians would receive God’s Spirit, which would help them overcome sin and would lead them into all truth: "I will ask the Father, and he will give you another Helper... It abides with you, and will be in you.... [and] shall teach you all things, and remind you of all things which I said to you" (John 14:16–17, 26, Wilson’s New Emphatic Version).
To properly answer the question, we should first look at how we receive the Holy Spirit. The Apostle Peter’s explanation was simple, direct and profound: "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit" (Acts 2:38). To repent means to "turn around" from the way of selfishness and self-will, and to go the other way—God’s way. Repentance is a heartfelt change from rebellion against the Lord’s instruction to unconditional surrender to God.
A truly repentant person will desire to live by "every word that proceeds from the mouth of God" (Matthew 4:4). Scripture enjoins that one who has repented must then be baptized. This important requirement reflects the repentant person’s inward faith in Christ’s sacrifice and resurrection, and submission to the true Gospel of the Kingdom of God (Romans 6).
This raises an important related point: can an infant make the deliberate and rational choice, and change of heart, that precedes baptism? Obviously not! Just as an adult’s baptism is not valid unless he or she has come to genuine repentance, an infant child is certainly not capable of mustering the faith and repentance that Scripture requires.
After baptism, one receives God’s Spirit through the laying on of hands (Acts 8:17). God’s Spirit allows the Christian to "walk in newness of life" (Romans 6:4). But what does that mean? The Apostle Paul explains that "the love of God hath been shed abroad in our hearts through the Holy Spirit which was given unto us" (Romans 5:5, ASV).
Indeed, this supernatural, divine love is described as the fruit of the Spirit, which is abundant in the life of true Christians (Galatians 5:22–23). The Apostle John explains how this love is manifested: "This is the love of God, that we keep His commandments" (1 John 5:3). Indeed, it is through God’s Spirit within that a Christian grows ever more able to keep God’s commandments.
As John wrote: "Now by this we know that we know Him, if we keep His commandments.... Whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him" (1 John 2:3–5). Even more directly, it says: "All who obey his commandments abide in him, and he abides in them. And by this we know that he abides in us, by the Spirit that he has given us" (3:24, NRSV).
The commandments express love toward God and love toward neighbor (Matthew 22:37–40). Jesus said: "By this all will know that you are My disciples, if you have love for one another (John 13:35). John expands on this: "We know that we have passed from death to life [through receiving God’s Spirit], because we love the brethren" (1 John 3:14). Such love is demonstrated not by mere well-wishing, but by actual deeds (vv. 18–19).
This love deepens as one grows more and more like Jesus Christ, through the indwelling of God’s Spirit, as the very nature of the Godhead is formed within us (Philippians 2:5; Galatians 4:19)!
This is how we know that God’s Spirit is working within us, preparing us for eternal life in the Kingdom of God!