The term 'substitutionary atonement' has been used within Trinitarian, Protestant, Reformed Baptist circles for some time, and it is those I wish to examine in this question.

But the word 'substitute' does not translate a Greek word found in scripture, nor does the term 'atonement'. The word 'atonement' is found once in the KJV, Romans 5:11, but it is a clear mis-translation of the word καταλλαγην, katallagen, in all other places rendered 'reconciliation'.

Both words are vague in meaning. Nor does 'substitute' or 'substitution' convey a concept that the apostolic epistles express, the emphasis of the doctrine of Christ being union with Christ (in his sufferings, in his death and in his resurrection) rather than some kind of 'exchange' (another word never found in Greek except μετηλλαξαν, metellazan, in Romans 1:26).

The word 'atonement' has a weak etymology and an ill-defined concept, its meaning a loose 'at-one' derivation and its application being a very general and overall term for the both the sufferings and death and resurrection of Christ that is never found in the greater precision of the apostolic writings.

What exactly is being conveyed by the term ? When was the expression first coined ? What error was being resisted by the introduction of this couplet ?

Again, I am looking for a response in regard to Trinitarian, Protestant, Reformed and Baptist usage of the terminology.