In Rom. 3:25 and Heb. 9:5 (A.V., "mercy-seat") the Greek word hilasterion is used. It is the word employed by the LXX. translators in Ex. 25:17 and elsewhere as the equivalent for the Hebrew kapporeth, which means "covering," and is used of the lid of the ark of the covenant (Ex. 25:21, 30:6). This Greek word (hilasterion) came to denote not only the mercy-seat or lid of the ark, but also propitation or reconciliation by blood. On the great day of atonement the high priest carried the blood of the sacrifice he offered for all the people within the veil and sprinkled with it the "mercy-seat," and so made propitiation.
In 1 Jn. 2:2, 4:10, Christ is called the "propitiation for our sins." Here a different Greek word is used (hilasmos). Christ is "the propitiation," because by his becoming our substitute and assuming our obligations he expiated our guilt, covered it, by the vicarious punishment which he endured. (Comp. Heb. 2:17, where the expression "make reconciliation" of the A.V. is more correctly in the R.V. "make propitiation.")