Final answer:
The teaching that ordinations are only valid when performed by a bishop in apostolic succession is a key principle of Roman Catholicism and Eastern Orthodoxy, while Reformers like Martin Luther argued that all baptized Christians have spiritual authority and can perform priestly duties if necessary.
Step-by-step explanation:
The teaching that ordinations are only valid if performed by a bishop who is a successor to the apostles is a principle stemming from the ecclesiastical offices and practices within certain Christian traditions, notably Roman Catholicism and Eastern Orthodoxy. This belief is based on the idea of apostolic succession, the uninterrupted transmission of spiritual authority from the apostles through successive popes and bishops, which is thought to be necessary for valid sacraments and holy orders. This concept was challenged during the Reformation, most notably by Martin Luther who asserted that all baptized Christians are priests and possess spiritual authority. He argued that the office of the priesthood should be seen as a function of service rather than an indelible spiritual character that sets ordained individuals apart from other Christians.
Luther's arguments were a reaction against the ecclesiastical hierarchy and the practices like the sale of church offices (simony) and the imposition of mandatory celibacy for priests. He promoted the idea that in cases of necessity, any Christian layperson could perform priestly duties, thus removing the strict necessity for apostolic succession. This position was a part of broader reforms during the period that sought to put church authority back into the hands of local congregations and align church practices more closely with New Testament teachings.