We Orthodox like to state that we know where God's grace is but we dont know where it isnt.
This is often taken too far as if it's a dogmatic or even patristic point, when really it doesn't predate the late 20th century.
Are you saying that that is not an accurate characterization of Orthodox thinking?
Well, are we to be agnostic on sacramental grace operating amongst Mormons? I think not.
Is that saying supposed to pertain to the sacraments? And what is meant by God's grace? I thought that God's grace could operate anywhere, including among Mormons. But maybe I don't have a good grasp of what grace is.
God's Grace is His Energies and is certainly present everywhere at all times, and, being God, He can certainly do anything He wants. However 'sacramental Grace' is specifically referring to that subset of God's activity where God has made (or has empowered the apostles and bishops of the Church in His name to make, to 'bind and loose',) specific promises that if man does x, He will do y.
Thus, for example, Christ saved the thief on the Cross by simple fiat. But He has also made a promise that if an individual undergoes the sacrament of Baptism, He will cleanse their sins and unite them to the Church. We cannot assume that God won't save others in the same way as He did the thief if He chooses to do so, but we also cannot assume that He *will* choose to do so, and no individual should *expect* or can demand of God that they be saved in this way. By contrast, God has made a commitment with regard to baptism, and so we can be certain that if we receive this sacrament, we will receive the promised works of Grace (remission of sins, addition to the Church). And the same goes for the Grace associated with Chrismation, Ordination, Confession, etc--and most particularly the Mystery of Communion with its transformation of common bread and wine into the Body and Blood of Christ Himself.
'Sacramental Grace', accordingly, is not present everywhere at all times, because the commitment of this Grace was given to and through the Church. Just because someone (pagan, heterodox or even Orthdoox layman) picks up bread and wine and says the words of Institution over them does not set any requirement on God to respond by actually transforming the items. On the other hand if a properly ordained minister of the Church performs the sacrament, God has made the commitment that He will perform the transformation.
Thus, one of the points of discussion that comes up in discussing schism is at what point schismatics lose 'sacramental Grace'--not that they ever lose access to God's free offerings of Grace, but in separating from the Church they do lose the promises of specific Grace possessed by the Church.