It is very important for our salvation that our God is a trinity. The word "trinity" is not used in the Bible, but that God is three in one is everywhere in its pages. The Father creates us, the Son redeems us, and the Spirit sanctifies us. All three of these things are necessary for the Christian life.
Matthew 3:16-17. When Jesus is baptized by John in the river Jordan, he comes up immediately from the water, and the heavens were opened, and he saw the Spirit of God descending as a dove and lighting on him. He has been "christened" for his office as redeemer. A voice out of the heavens--the Father--says that Jesus is his beloved son, in whom he is well pleased. The Father is the mastermind behind the plan of salvation, and he is pleased with the human vessel to carry the plan out. Jesus is the redeemer of the world, the Word of God made flesh, who dies for the sins of believers. The Holy Spirit applies redemption to the believers and carries out the cleansing of our sins, making them white as snow, like a dove.
Matthew 28:18-20. The Father has given the Son all authority in heaven and on earth. Jesus, the Son, takes that authority and passes it down to all believers, giving us the power to preach the gospel to all nations, making disciples. Baptizing them in the name of the Father and the Son and the Holy Spirit begins the regeneration process. Believers are preached to and become sanctified. The act of discipling is conceived by the Father, enabled by the Son, and impossible to accomplish without the Spirit.
John 14:16. Jesus tells his disciples that he, the Son, will ask the Father, and he will give his disciples the Spirit of truth to help us. Once again, the Father is the mastermind, Christ is the agent of change, and the Holy Spirit is the substance of enlightenment. In this case, he will lead his children in all truth, teaching all things, opening the scriptures to understanding, and testifying about Jesus to the world (John 15:26).
John 16:5-11. The Spirit also keeps in our minds the trinity. Not only does he convict the world of sin, but he alerts the world to the righteous solution for sin, because Jesus is with the Father, interceding for us. The Spirit also promises a day of judgment and that there is only a short time left, time enough for us to receive faith in God. Faith is not just believing in God, but a thorough understanding of the gospel, and that understanding involves knowing the trinity. The gospel doesn't make sense unless the roles the trinity plays make sense.
Salvation involves the complete trinity. If you don't have the Spirit, you don't have the Son. If you don't have the Son, you don't have the Father. Not believing in one means you don't believe in any of them. Not being aware of the work of one means the other two are not working in your life, either. How do you know if God is active in your life? The Holy Spirit bears fruit in the believer's life. If you can see the fruit of faith, then you know the Holy Spirit is sanctifying you, and you know that the Son is interceding for you, and you know that the Father has elected you. If you practice the deeds of the flesh, then you are outside the trinity. The deeds of the flesh, according to Galatians 5, are immorality, impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing. The fruits of the Spirit are love (as Christ loved us), joy (even in persecution), peace, patience, kindness, goodness, faithfulness, gentleness, self-control. Examine yourselves. These fruits are not difficult to discern. Is the trinity at work in your life? Has the Father created a new person? Has the Son spilled his blood on your behalf? Is the Holy Spirit sanctifying you? Are you bearing the fruit?