Peter gets praised by Jesus for perceiving early on, even before the Holy Spirit has descended, that he is the Christ, the son of the Living God. John the Baptiser gets there even earlier with an equivalent understanding at the very beginning of Jesus's ministry. Behold, the Lamb of God, who takes away the sin of the world. John 1:29-31
Once one has learned this and sung it, it is hard not to sing along. And is always the case with Handel, each part is quite sure that it is the one George intended to be dominant.