Complex sentence: For Irenaeus the perfection of Christ originates from his being "The Word", i.e. the Logos which pre-existed as Christ in perfect form, untouched by sin: because he was the first, he could achieve perfection.
Split into simpler sentences: For Irenaeus the perfection of Christ originates from his being "The Word", i.e. the Logos which pre-existed as Christ in perfect form, untouched by sin. Because he was the first, he could achieve perfection.