The tradition of the wedding kiss originated in Ancient Rome. It sealed the engagement contract and was legally-binding, symbolizing both parties acceptance of the contract. Some say this is where the phrase "sealed with a kiss" came from.
The kiss was so binding that if either the Roman bride or groom died before the actual wedding ceremony, but had already kissed with the engagement contract, the living one got to keep the wedding gifts!
Later in the Church of England, the couple could only after the bride first kissed the minister, a symbol of good luck.
It was also an early Christian belief that the bride and groom swapped souls during the wedding kiss. It is still part of weddings around the world, in many different religious ceremonies, and many continue to consider the kiss to physically unite the couples' souls.