St. Athanasius the Great (reposed in 298 AD) said "The Son of God became man, that we might become god"

This is in reference to, for example, 2 Peter 1:4 where he talks about us being partakers of the divine nature, which is only possible through Christ and His Grace. This is Theosis. 

We are already made in His Image. Theosis is a call to strive to become like God in likeness. What that means is it is a call to follow Christ’s commandments to love the Lord our God with all our heart, soul, mind, and strength. Secondly, to love our neighbors as ourself. If we do these truly do these things, we unite ourselves to Christ.

Theosis is indeed apostolic tradition, and has nothing to do with Hesychasm. Many saints who were not Hesychasts achieved Theosis. 

And Hesychasm was very present in the early church, especially in early monasticism, their lives can read about here: https://stanthonysmonastery.org/products/the-lives-of-the-great-desert-fathers