Leyline of Sanctity

The free turn-zero clause is what makes this more than a four-mana hexproof enchantment. The leyline cycle's shared rule (begin the game with it on the battlefield from your opening hand) means the card frequently costs nothing and arrives before the first land drop, sealing your face off from every burn spell, every discard spell, every targeted removal aimed at your dome, and every combo piece that needs you as a target to win. That last category is the real job: granting yourself hexproof shuts down the entire family of "target player loses the game" and "target opponent" effects, which is a different kind of protection than warding a creature. It does not stop board wipes, sacrifice effects, or anything that says "each player," so it is a wall against pointed interaction, not a fog against everything. The constraint that pays for the turn-zero discount is its narrowness and its fragility: it does only one thing, it can be answered by enchantment removal, and drawing it past your opening hand makes you pay the full white-heavy cost for a hate piece you would rather have had for free. White has a long tradition of these prophylactic enchantments that ask nothing on the turn they matter and everything on the turn they arrive late; this is the version that protects the player rather than the permanents, which is precisely why it has lived in sideboards for over a decade against decks that need you to be a legal target.