Have you noticed how ridiculously hard it can be these days to get what you want out of a bottle, a jar or a box? Whether it's food, medicine or simply a new toy, the glues, plastic and wires of packaging, or the wickedly engineered variety of tamper proof seals and caps; the everyday act of opening these vessels has been rendered a challenge of biblical proportions.
The manufacturers of these various products design their containers and packages to be difficult to open because they want to ensure that the only person who gets what's inside, is the person for whom the product is intended. Sometimes packaging is intensely intricate to protect the contents during travel. Fair enough, but sometimes it's so hard to get into what you want that you wonder if it's going to be truly worth the effort.
In the case of food and medicine, tamper proof seals are used to protect the contents from contamination. To ensure it's purity. When you buy a drink for example, and break the seal as you unscrew the cap, you can be reasonably sure that the liquid inside the bottle is exactly what the producer intended you to consume. No hidden nasties.
Because God is holy, nothing impure or sinful can enter his presence and so, in a sense, Heaven is sealed against contamination, and entering it requires much more than any mere human attempt to break the seal. You cannot force your way into heaven either by sheer volume of good deeds or by an extreme effort of will power or wishful thinking. Your "goodness" will never be sufficient.
The only way in is through faith; faith in the redeeming power of Christ whose body was broken and blood shed that we might be able to have a relationship with God. It was our Maker's plan that the tamper proof seal of Heaven could only be broken by a violent and sacrificial act. Jesus broke the seal by his death and resurrection. He made a way for us to enter the perfectly pure peace of Heaven.