Irenaeus preaches against the most common heresy that creeped into the church early on: gnosticism. There are many forms of gnosticism, but the one Irenaeus decries is the Valentinian kind. These people follow Valentinus.
Let me say, it is confusing, but I'll try to explain it.
In the highest heavens there is the most excellent being. These days he is called Bythus, but he has other names. Bythus had a wife named Sige. They begat male and female pairs of twins. There are 15 pairs and together they are called the Ogdoad. All the individuals are called Aeons.
Aeons wish to honor Bythus, so they make their own aeon pairs. This group of thirty aeons is also called a Pleroma.
For 30 years, Jesus the Savior kept silent while these aeons ruled the world. Jesus is also called Monogenes, and he sprang from Bythus unknown to all except for the aeon Nous. Ultimately, one of the youngest aeons, Sophia, tried to create a child with Bythus. She wanted passion apart from her twin aeon. Because of this she fell from heaven and somehow had Jesus.
These people also consider Jesus Christ and the Holy Spirit to be aeon twins.
Somewhere in there, matter is also evil, and it was created through Monogenes who was the product of Sophia and Bythus. He is the evil Demiurge who is God of the Old Testament, and Jesus came to save the world and the aeons from him.
So basically, 1. Jesus and the Holy Spirit are less than God.
2. God the father is a demiurge
3. How did Jesus get here again? I'm confused.
4. How insulting to say that all the matter and the universe around you with its beauty is an evil gift from the OT God!
5. This is not Christianity but its ideas crept into the church teaching the way they do today.
This lives on today. All the names are changed and might even be real people from time to time. But if you are in a place where Jesus is merely God's power and not God, then you are drinking the Valentinian Gnostic kool-aid. If the God of the OT and Jesus are separate beings, you might be a gnostic.
Do not be deceived. God created a good world. Man corrupted it with Sin. God sent his Son Jesus as a gracious gift to save the whole planet and redeem his people back to him again. It's like night and day different from Gnosticism. Matter is not the problem. God did not cause evil. We caused it by disobeying God. Now he sent Jesus not accidentally but planned it from before creation to save humans and matter from the evil of treason against our Creator.