There seems to be a logical sequence in the Nicene Creed of 325:
But the revised creed of 381 omits the phrase “from the substance of the Father.” That seems to break the link between "begotten" in (1) and homoousios in (3). Any idea why and what the implications are?
We believe in one God, the Father almighty, maker of all things visible and invisible; And in one Lord, Jesus Christ, the Son of God, begotten from the Father, only-begotten, that is, from the substance of the Father, God from God, light from light, true God from true God, begotten not made, of one substance with the Father, - Early Church Texts
The phrase 'from the substance of the Father' appears to be given as an explanation of what 'only-begotten' entails. The later phrase 'of one substance with the Father' appears as an extrapolation or clarification of 'begotten not made'.
Since it is a biblical axiom that 'like begets like' the phrase 'from the substance of the Father' doesn't add anything that is not included in saying 'God from God' and therefore its removal doesn't 'skip a step' in the logical progression:
"God from God, light from light, true God from true God, begotten not made, of one substance with the Father
Since the later creedal revision adds in much more about the Holy Spirit and, since creeds by nature of intent prefer to remain as simple and clear as possible for the purposes of memorization, redundant phrases were edited out.
From an early stage baptism was preceded by catechetical instruction. We may suppose that in time this developed along standard patterns. We cannot say exactly when a creed in the formal sense was first associated with such instruction but Hippolytus in The Apostolic Tradition demonstrates the resultant purpose of such catechismic, creedal teaching some 110 years before the first Nicean council:
‘And when he who is to be baptised goes down into the water, let him who baptises lay his hand on him saying thus. “Dost thou believe in God the Father almighty?” And he who is being baptised shall say. “I believe”. Let him forthwith baptise him once, having his hand laid upon his head. And after this let him say, “Dost thou believe in Jesus Christ, the Son of God, Who was born by the Holy Spirit from the Virgin Mary, Who was crucified under Pontius Pilate and died, and rose again on the third day living from the dead, and ascended into the heavens, and sat down at the right hand of the Father, and will come to judge the living and the dead?” And when he says. “I believe”, let him baptise him a second time. And again let him say. “Dost thou believe in the Holy Spirit, in the holy Church, and the resurrection of the flesh?” And he who is being baptised shall say, “I believe”. And so let him baptise him the third time.’ - The Apostolic Tradition (from chapter 21)
