'Magna Veritas' means 'Great (is) Truth'. The usual occurence of the phrase 'In Nomine' is in 'In nomine Patris, et Filio, et Spiritui Sancti', that is, 'In the name of the Father and of the Son, and of the Holy Spirit'. While the original IN appears to have been something of a stab at Catholicism, the SJG version is a fairly reverential, not to mention clever and genuinely adult treatment of the material.