Since this question keeps on being repeatedly asked by many, here is an answer I originally posted back in 2013:
To be a child of God means to be an adoptive member of the Divine Family.
When we become children of the living God by taking steps to rediscover our true heritage and giving the Almighty Father our fiat with sincerity of heart, in essence and in fact, we become adoptive members of the Divine Family. The Family – the Father, the Son and the Holy Spirit and, yes, even the Mother* – take us in. They take us into Their Hearts and envelop us in Their bosom, showering us with all we need to become gods by grace (2 P 1:4; Ps 82:6).
We are creatures: made, not begotten. Clay in origin; no more, no less. But in our humanity, in its very weakness, we have been raised higher than the angels by the grace of God. And if we so desire, with humility, we can receive the fullness of the Father’s divine life in Jesus Christ (Paul VI, 1964). That is the reason we have all been created in His image, to attain fully to His likeness: to become deified not solely in patria, but in via and in patria (Dionysus the Aeropagite, EH 1.3, PG 3.376a; Gregory of Palamas, 1983; Gross, 2003; Lot-Borodine, 2011; Williams, 1999).
This is what it means to live in the Divine Will, to be clothed from head to toe with it (Piccarreta, 1899-1938). It is the coming of the Holy Spirit in His fullness and taking us up into Himself (Bartolo-Abela, 2012; Seraphim of Sarov, 2010), the coming of the Kingdom in our hearts and souls. All depends upon whether we want be part of the Family or not – whether we desire to live with and in Them or not, and have Them live with and in us, in return. It is our free-will choice.
*On the Mother of God
Only God the Father, the Son and the Holy Spirit are divine by nature. Mary, the Mother of God, is human by nature, not divine. But as Mary was sinless in perpetuity through the special graces bestowed upon Her by the Holy Trinity, She was without impediment and deified fully by God (Gregory of Palamas, 2005). Mary was the perfect human creature who partook fully, not partially, of the divine nature (2 P 1:4), becoming a god by grace (Athanasius of Alexandria, De Incarnatione 54.3; Augustine of Hippo, Sermon 166.4; Bartolo-Abela, 2013; Jn 10:34-35; Paul VI, 1965; Ps 82:6; Thomas Aquinas, Summa Theologica 22.214.171.124).
Bartolo-Abela, M. (2012). The icon of the Divine Heart of God the Father: Apologia and canon, p. 83.
Bartolo-Abela, M. (2013). The Divine Family: Experiential narratives, p. 79.
Gregory of Palamas. (1983). The triads in defense of the holy hesychasts (J. Meyendorff, ed.).
Gregory of Palamas. (2005). Mary the Mother of God: Sermons by Saint Gregory Palamas (C. Veniamin, ed.).
Gross, J. (2003). The divinization of the Christian according to the Greek Fathers (P. A. Onica, trans.).
Lot-Borodine, M. (2011). La deification de l’homme selon la doctrine des Peres Grecs.
Paul VI. (1964). Lumen Gentium.
Paul VI. (1965). Dei Verbum.
Piccarreta, L. (1899-1938). The Kingdom of the divine fiat in the midst of creatures: The call to the creature to return to the order, to the place and to the purpose for which it was created by God (Vols. I-XXXVI).
Seraphim of Sarov. (2010). On acquisition of the Holy Spirit.
Williams, A. N. (1999). The ground of union: Deification in Aquinas and Palamas.