We do not only need to believe in the Son of God, we must also believe in the One who sent Him, i.e. God the Father.
We cannot please God if we do not believe He exists or rewards those who seek Him.
Can we be saved if we do NOT believe in the Trinity, One God in three Persons? Of course not.
That would be like saying I believe in one third of God, or I believe in two thirds of God, but I do NOT believe in the Triune God.
Accepting (for no one can really understand) the Trinity is a theological development that should happen after getting born again. We must believe in Jesus Christ (which includes His sacrifice on the cross for our sins and His resurrection from the dead). That is all. Salvation is that simple.
There is no verse that says we must believe in a trinity or believe in God the Father, God the Son, and God the Holy Spirit as a triune God, to be saved, even though this formula is true. Many things about God are true, but salvation is not accomplished by embracing theological concepts that are difficult to comprehend.
What scripture actually directly says about getting saved:
And they said, “Believe in the Lord Jesus, and you will be saved, you and your household.”
For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life.
Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
Jesus said to him, “I am the way, and the truth, and the life. No one comes to the Father except through me.”
Truly, truly, I say to you, whoever believes has eternal life.
Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him.
I told you that you would die in your sins, for unless you believe that I am he you will die in your sins.
Jesus answered them, “This is the work of God, that you believe in him whom he has sent.”
But these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
And Peter said to them, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit.”
For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day.
Because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved.