Baptism became an important part of believing after Jesus Christ himself gave the instruction that it should be done.
"Go into all the world and preach the good news to all creation, whoever believes and is baptized will be saved, but whoever does not believe will be condemned." Mark 16:15,16.
The word "baptism" from the Greek 'baptizo' means 'to dip, to immerse' , it is complete immersion in water. Baptism is symbolic of taking part in Christ's death, hence the complete 'burial' in water, which in turn symbolically washes away our sins and enables us to start afresh. The apostle Paul wrote a very good message on this subject to the Romans which I suggest you read, Romans chapter 6, a least the first 10 verses. So basically, if we have not been fully immersed in water, then we have not taken part in Christ's death, or had our sins washed away.
In the New Testament, baptism is always accompanied by belief and repentance.So the point arises that baptism occurs as a result of belief.
Belief in what?
In Mark 16:15,16 it says "Go into all the world and preach the good news (gospel) to all creation "
But Paul, when he wrote to the Galatians, ch3:8,9 said "The Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: All nations will be blessed through you."
So belief in Jesus alone is not sufficient, belief involves God's whole plan which is summed up in the promises to Abraham.
The Bible answer to your question must be that baptism by immersion is essential to salvation, but it must be accompanied by belief in what God has told us in the whole of the Scriptures.
Just the act of baptism will not assure us salvation, it means that, in the analogy of the apostle Paul, we can start the race and we have to keep on working throughout our lives to ensure that we get the prize, eternal life.
Hebrews 3:14 tells us, "We have come to share in Christ if we hold firmly till the end the confidence we had at first".
James 1:12 reads, "Blessed is the man who perseveres under trial, because when he has stood the test, he will receive the crown of life that God has promised to those who love him."