If one believes in Jesus, they believe in the Trinity - Father (God), Son (Jesus), and the Holy Spirit.
I read this somewhere which explains it in a way that made sense to me.
"Without Christ….there is no salvation. The Book of Hebrews explains that Jesus is the mediator, the executor, of The New Covenant (The New Testament). The word mediator comes from the verb "to mediate" which means a mediator is one who stands between two parties or two factions needing to be reconciled eternally. Ceremonial is a law which was fulfilled by Christ, which Christ "took out of the way, nailing it to his cross."
During Jesus' ministry, Jesus spoke of His coming death. "Destroy this temple," Jesus said, referring to His body, "and I will raise it again in three days." Jesus also declared that the Father would raise Him up on the third day. In Romans 1 we read that the Spirit of God declared Jesus to be the Son of God with power "by his resurrection from the dead." Hebrews 9:14 tells us that Father, Son and Holy Spirit all had a part. The Father received the offering of the Son through the Holy Spirit. Jesus Christ, who is God, the blameless and sinless Son, offered as the Lamb of God, without spot and without blemish. The redemptive price was paid by the Son to the Father through the Spirit. Christ through His death and resurrection, every legal hindrance has been met and satisfied. There is nothing that can keep me from assurance except my own faith. Its not about thinking our way into salvation or to reason our way into salvation. The only way to get in is to believe Him with my heart which is sealed by the Holy Spirit. Hebrews 9:15 For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritancenow that he has died as a ransom to set them free from the sins committed under the first covenant. Hebrews 13:20 May the God of peace, who through the blood of the eternal covenant brought back from the dead our Lord Jesus, that great Shepherd of the sheep. 1 John 2:25 And this is what he promised useven eternal life. Jude 1:21 Keep yourselves in God's love as you wait for the mercy of our Lord Jesus Christ to bring you to eternal life."