The parties have different versions of the agreement. This is often a problem with oral agreements. The parents may disagree about the terms of the original agreement. A court will not enforce an agreement if it cannot determine what the parents originally agreed to. The agreement is too old.
Courts often hesitate to bind either parent to an agreement that was made many years in the past.
Courts don't want to curtail First Amendment and parenting rights. As previously mentioned, courts are loathe to tramp on an individual's First Amendment or parenting rights. Not all courts dismiss religious upbringing agreements, however. For example, in September 1999, an Indiana court ruled that a divorce settlement's terms governing the religious upbringing of the children was binding on both parties. The short of all this is that, if you enter into an agreement about the religious upbringing of your children, it stands the best chance of being enforced by a court if it is in writing, very detailed and no more than a couple of years old.