Every week, we sign the Trisagion hymn before the confession of sin. Trisagion means “thrice holy,” and the hymn is theologically rich with deep historical roots.
The Trisagion is a plea for mercy to the Holy Trinity. In the ancient church, there was actually a group that sought to change the hymn. After “Holy Immortal” this group inserted “Who was crucified for us…” The orthodox Christians rejected this addition because it included the whole Trinity in the sufferings of Jesus in a way that would have compromised God’s impassibility. 
John of Damascus (676-749) wrote,
“We understand the ‘Holy God’ as referring to the Father, and yet we do not restrict the appellation of divinity to Him alone, but recognize the Son and the Holy Ghost to be God, also. The ‘Holy Strong’ we take as referring to the Son, yet we do not strip the Father and the Holy Ghost of their strength. And the ‘Holy Immortal’ we apply to the Holy Ghost without excluding the Father and the Son from immortality, but understanding all the divine attributes as referring to each of the Persons.” (On the Orthodox Faith, III.10)
The hymn is alluded to by Athanasius (296-373) in his comments on Matthew 11:27, and described in the records of the Council of Chalcedon (451). This means that it was probably being used in Christian liturgy by at least the 4th century, but likely even earlier. The hymn was a mark of Trinitarian orthodoxy, and when we sing it we are singing the song of saints and martyrs past, who upheld the faith in the face of strong opposition.
 Impassibility is the doctrine that God is immune from suffering. This is not to say that God is apathetic, but that He is not overwhelmed/overcome by the world. Christians confess that one from the Trinity (Jesus) suffered by assuming human nature, but deny that the Father and the Spirit also suffered together with Jesus.