Note 17 at 1 Th. 3:13: In this verse Paul is saying that their hearts being established in holiness is the end result of their love abounding, which he spoke of in the previous verse. Holiness is a by-product of love, not the means by which we receive love.

Holiness isn't something that God does. It is His very nature. It's Who and What He is. All the moral laws revealed in scripture are just reflections of who God was before scripture was ever written. Likewise, our holiness is a product of who we are and not what we do.

Through the new birth, every believer received a righteous nature (see notes 15-18 at Eph. 4:24, p. 1123). Our actions didn't produce the new nature, but the new nature will produce actions if we allow it to dominate our thinking. As we let the love of God which was deposited in our born-again spirit flow through us, holiness becomes a fruit, and not a root of our salvation.

Note 18 at 1 Th. 3:13: Paul now mentions the second coming of our Lord Jesus Christ and expounds on this further in 1 Thessalonians 4:13-5:11.