Essential, fundamental, vital, cardinal mean so important as to be indispensable.
Something is essential which belongs to the very nature or essence of a thing and which therefore cannot be removed without destroying the thing itself or its distinguishing character.
Something is fundamental upon which everything else in a system, institution, or construction is built up, by which the whole is supported, or from which each addition is derived and without which, therefore, the whole construction would topple down.
Something is vital which is as necessary to a thing’s existence, continued vigor, or efficiency as food, drink, and health are to living things.
Something is cardinal upon which something else turns or hinges or actively depends; thus, the cardinal virtues (prudence, fortitude, temperance, justice, and sometimes, patience and humility) are not, in Christian theology, the highest virtues (which are the Christian virtues faith, hope, and charity), but they are fundamental and without them moral progress would be impossible.