The truth about Heaven. There is a Heaven. If there were no Heaven, then God lied to us. Since God can not lie, being the fullness of truth, there must be a Heaven as He promised.
Heaven like Hell is eternal. You can not be kicked out of Heaven for bad behavior after you get in. There will be nothing you desire more, so no one will want to leave. The key is getting there in the first place.
The images of Heaven include a banquet, a great city, a wedding feast. We can not begin to imagine what Heaven is really like. What constitues "perfect happiness"? I once pointed out to a golfer friend that Heaven is not a golf course. What happiness could there be in always making a hole-in-one? Everyone would be doing it. There would be no challenge. You could beat anyone or have anything to brag about. Our images will fall very short of what God has in store for us. "Eye has not seen, nor ear has heard the wonders our God has for us."
Getting to Heaven involves both God's Mercy and Justice. In His Mercy He makes it easy for us. All we have to do is repent of our sins and desire to follow Him. In His Justice He requires us to repent of our sins because nothing sinful can entered into the presence of God. In His Mercy He even accepts an imperfect act of contrition. In His Justice He allows us to use our Free Will to reject His offer of Mercy.
The Church teaches us the meaning of Sin so that we can be clear about what is expected of us. The Church teaches us about God's Mercy and Justice so we don't commit the sin of presumption and fail to repent. We can't say we didn't know!