4 But this [Father] is the Maker of heaven and earth, as is shown from His words; and not he, the false father, who has been invented by Marcion, or by Valentinus, or by Basilides, or by Carpocrates, or by Simon, or by the rest of the “Gnostics,” falsely so called. For none of these was the Son of God; but Christ Jesus our Lord [was], against whom they set their teaching in opposition, and have the daring to preach an unknown God. But they ought to hear [this] against themselves: How is it that He is unknown, who is known by them?
For, whatever is known even by a few, is not unknown. But the Lord did not say that both the Father and the Son could not be known at all (in totum), for in that case His advent would have been superfluous. For why did He come hither? Was it that He should say to us, “Never mind seeking after God; for He is unknown, and you shall not find Him;” as also the disciples of Valentinus falsely declare that Christ said to their Æons? But this is indeed vain. For the Lord taught us that no man is capable of knowing God, unless he be taught of God; that is, that God cannot be known without God: but that this is the express will of the Father, that God should be known. For they shall know Him to whomsoever the Son has revealed Him.
Source: Adversus haereses (New Advent)