I shall try to answer your questions in order.
- What scripture confirms papal infallibility?
- Doesn't Ephesians 2:20 negate such a premise?
- Is our 'rock' not the same as Peter's, our confession of faith - You are Christ, Son of the living God?
1. If Jesus Christ established one permanent Church to which we should all belong, it must always be possible to belong to it without fear of being led astray in matters of faith and morals. Christ could not command us to hear and obey the Church if His Church itself were a guide liable to be mistaken. He said: "You are Peter and on this rock I will build my Church. And the gates of the underworld can never hold out against it" (Matt 16:18). So, Jesus Christ made St Peter the foundation of His Church, that is, the guarantor of her unity and strength, and promised her a duration that will not pass away. However, the unity and solidity of the Church is not possible without the right Faith. So, St Peter is also the supreme teacher of the Faith. As such he must be infallible in the official promulgation of Faith, in his own person and in his successors since by Christ's decree the Church is to continue for all time. So, it is Catholic doctrine that the infallibility as well as the authority of the Church finds its last court of appeal in the Pope as visible head of the Church.
2. No. When St Paul wrote to the Ephesians: "You are part of a building that has the apostles and prophets for its foundations, and Christ Jesus himself for its main cornerstone", he was stating exactly what we state in the First Eucharistic Prayer at Mass, ". for all who hold the catholic faith which comes to us from the Apostles"; but this in no way takes away from the primacy of St Peter among those chosen men.
3. Again, No. Our Lord is not referring to St Peter's expression of faith as the rock (so that it could be compared with ours), but to St Peter himself. It seems that neither the Greek word petros nor the Aramaic kepha was used as a person's name before Jesus conferred it on St Peter to indicate the part he was to play in the foundation of the Church.
I hope these replies are helpful.
May God bless you with a very happy and holy Christmas!
Fr Jordan O.P.