Suppose one of your friends challenged you in a bicycle race along the rim of the Grand Canyon. To make it to the finish line you needed to jump over the canyon to the other side. Now if you've ever been to the Grand Canyon you know that's a looong way. But everyone there insists that they have the best bikes and they can make it. So the race starts and off you go. The first one is on his way to the canyon rim. He sets the gears on his bike and speeds rapidly to the edge. Off he goes, flying through the air. It looks good. Then without warning he starts to loose altitude. Down he goes disappearing from site.
The next one takes off through the air with similar results. He has made it a little farther but still falls way short of the other side.
One after the other each biker launches off the rims edge only to fall to the same fate. Some make it a little farther than others, but still they all fall short. It was impossible for any of them to land safely on the other side.
This is the way it is between man and God. We want to go to Heaven but there is a gap between the kingdom of God and man. Each person tries hard to fill that gap. Some do lots of good deeds such as helping their neighbors, going to church, giving money to the homeless, etc. Some may feel they have done more to get them farther. Maybe they give large sums of money to charities or donate many hours volunteering. Some may go a little farther across that gap than others but none succeed getting to the other side.
The problem is even though man knows he's not perfect he figures God will at least accept his best. But God does not grade on a curve. He has a standard to uphold and that standard is perfection! Jesus said 'Be ye perfect even as your Father in Heaven is perfect." No matter how much good you or I do we cannot cover up the fact that we are not perfect.
So if God demands perfection from us in order to enter Heaven and none of us meet that standard then there must be another way. That way is through Jesus Christ. He lived the perfect life for us, took the punishment for our sins (that we deserved) on the cross and paid the debt of our sin in full. By trusting what Jesus did for us on the cross we can enter Heaven clean from sin in God's eyes.