What is Gnosticism and what modern day religion does it play the biggest role in?

What is Gnosticism and what modern day religion does it play the biggest role in?

Basically Jesus told secret truths to some disciples but not to others. It has been embraced by the new age type "Christianities" along with other ideas rejected by mainline Christianity.

I don't really know whether it has influence in the periphery of Christianity or in Secret Societies.

Gnosticism is an ancient esoteric religion that based salvation, or eternal life, on obtaining "secret knowledge" of a divine spirit-god. They condemned all physical matter and claimed it to have been created by a lesser, evil demure. The ancient Gnostic's claimed to possess that knowledge and promised to share it with initiates. They had a rather annoying habit of adopting villains from other religions and stories then rehabilitating them to spin them into possessing that same secret knowledge. They took the story of Cain and Abel and changed it into Cain obtaining nosies from Abel, put Judas in the position of obtaining nosies from Jesus, and created a list of "secret sayings" they attributed to Jesus then putting them in the mouths of his apostles.

There are some Gnostic's nowadays but none whose traditions reach as far back as the originals. It's more of a "new-age" movement as someone said above. The stories of Jesus found in the Koran are drawn from gnostic heresies, and one might compare Freemasonry to gnosticism but only to the point of keeping their rituals secret from the outside world.

There are some well over 3,000 sects in Christianity. Buddhism has even more than that.
Gnosticism has been around much longer than the "official" canon of the bible and has had many sects of its own. The recently rediscovered "Gospel of Judas" is considered to be gnostic by at least one biblical scholar. Your best source is "The Nag Hammadi Library", probably the best commonly available collection of gnostic writings.
As far as modern-day religions, they all have an anti-gnostic bias. However, much as they hate to admit if, there is a very gnostic slant to Paul's early (and known to be genuine) epistles. The later epistles are largely either questionable or clearly forgeries.
Blessings on your Journey!

Gnosticism first reared its ugly head in the days of Paul. They said that Christ was a created spirit being who was only one of many angels in a chain which you had to go through to get access to God. Gnostics denied that Christ was God and that he had a human body. Paul argues with them in Colossians. He states that Christ was begotten or born, not created. He counters that Christ was both human and divine. Paul goes on to say that Christ was eternal and created the world. "All things were created by him and for him."

The Gnostics incorporated Greek philosophy into their doctrine. They claimed that the Gospel had hidden knowledge that only certain people could know. Only they could interpret it. Paul says that none of this was done in secret. He and other Christian workers openly declared to the Colossians the Gospel. They left nothing out. The bottom line was Christ crucified to save us from our sins.

New Age beliefs would come closest to this. There are books out which name angels, which the Bible never did except for Gabriel and Michael. They have prayers that you're supposed to say to these angels for whatever you need. Colossians 2:18 states that we are not to worship angels. The First Commandment says to worship the Lord God and no one else.