The Hidden Altar; Zohar 80a

Abraham built two altars, one for the level that is revealed, and one for the level that is concealed.  Come and see that this is so, for it is written, 'Abraham built an altar for G-d that appeared to him' and elsewhere it is written 'Abraham build an altar there,' without any elaboration, or mention of 'G-d that appeared to him.'

Matok M'Dvash explains that the altar to revealed G_d is parallel to the outer altar of the Temple, where many forms of sacrifice were performed by various priests.  It is also parallel to Malchut, the lowest of the sefirot.

The altar to the concealed G-d is parallel to the inner altar of the Temple, where the high priest performed the incense sacrifice. and also the higher, less accessible sefirah of Binah.

I note that incense itself tends to conceal, while many of the sacrifices performed on the outer altar, such as the animal sacrifices, tended to reveal what is normally hidden.