I understand that this can become a charged issue for many Christians and that various Christian traditions over the ages have taught that Jesus’s death by itself was sufficient for atonement. Indeed, I believed similarly until I came across a scholarly work by David Moffitt. When we interact with various biblical data points, we will see that the Bible says something different about our topic. The belief that Jesus’ death was all needed for atonement has much biblical data for it. Here are a few:

John 1:29 ESV
The next day he saw Jesus coming toward him, and said, “Behold, the Lamb of God, who takes away the sin of the world!

The idea here is that of the sacrificial lamb in Jewish temple rituals. Of course, John would further clarify that this lamb was slain in Revelation 5:6. Together, the verses imply that the slaying of the Lamb equals the taking away of sins.