That Christ is interceding in heaven is explicated in the New Testament (e.g. Romans 8:34; Hebrews 7:25). This presents a conundrum to those who hold to forensic or penal substitution—why would Christ need to intercede for his people in heaven to ensure the application of his atonement if the legal price had already been paid?
The following comes from Darrin W. Snyder Belousek, Atonement, Justice, and Peace: The message of the Cross and the Mission of the Church (Grand Rapids, Mich.: Eerdmans, 2012), p. 249 n. 13:
To understand the heavenly intercession of the Son on our behalf as the propitiation of the Father, as Michael does, generates a significant problem of internal coherence for penal substitution. According to penal substitution, the primary purpose and effect of the death of Jesus was to propitiate the wrath of God on account of the sins of humanity. As it is written elsewhere, because Christ is “a priest forever” in heaven, he “always lives to make intercession” and is thus “able for all time to save those who approach God through him” (Heb 7:24-25). Heavenly intercession on our behalf is thus the ongoing vocation of the risen and ascended Christ. So, if the purpose and effect of the Son's intercession is to propitiate the Father's wrath, then the Son is continually doing in heaven at the throne what was to have been fully accomplished on earth at the cross. The cross would thus seem to have been ineffective, or at least incomplete, in accomplishing its primary purpose of saving humanity from divine wrath. Michael's [a Reformed apologist the author is responding to] interpretation of 1 John 2:1-2, although given in defense of penal substitution, effectively undermines it.