These were some of the same religious who had succeeded in having Jesus executed. They started thinking that His disciples might remove the body from the tomb and claim that Jesus had been raised from the dead.
To make sure this didn’t happen, they sealed the stone that had been placed across the entrance. They could have stretched a rope across the stone and sealed each end of the rope with clay or wax. If this seal were to be broken, they would know someone had tampered with he stone.
This may have been the way the entrance to the pit was sealed after Daniel was thrown to the lions by King Darius and his aides (Daniel 6:17).
MATTHEW 27:66 – They [the Jewish leaders] went, and made the sepulchre sure, sealing the stone, and setting a watch.