“The spirit of man is the lamp of the Lord, searching all his innermost parts.” Proverbs 20:27
What an incredible truth. Our spirit is meant to be the lamp of the Lord. A lamp requires oil to function (period). It cannot function without oil. Likewise, our spirit cannot function as God intended without the Holy Spirit.
When Adam and Eve sinned and disobeyed God the Holy Spirit left each of their spirits and they could no longer function as the lamp of the Lord. When we accept Jesus Christ as our Lord and Saviour, God the Father sends the Holy Spirit to once again indwell our spirit and enables it to function once again as the lamp of the Lord.

We cannot function as a Christian without the Holy Spirit. He is the one who produces the fruit of the Spirit in our lives.
“But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law. And those who belong to Christ Jesus have crucified the flesh with its passions and desires.” Galatians 5:22-24
He is the one who provides the nine gifts to enable us to minister effectively for God.
“Now there are varieties of gifts, but the same Spirit; and there are varieties of service, but the same Lord; and there are varieties of activities, but it is the same God who empowers them all in everyone.” 1 Corinthians 12:4-6
“For to one is given through the Spirit the utterance of wisdom, and to another the utterance of knowledge according to the same Spirit, to another faith by the same Spirit, to another gifts of healing by the one Spirit, to another the working of miracles, to another prophecy, to another the ability to distinguish between spirits, to another various kinds of tongues, to another the interpretation of tongues. All these are empowered by one and the same Spirit, who apportions to each one individually as He wills.” 1 Corinthians 12:8-11
Remember the Holy Spirit is the third person of the trinity make sure you do not grieve Him, nor quench His work in your life.
“And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.” Ephesians 4:30
“Do not quench the Spirit.” 1 Thessalonians 5:19