Rabbi Isaac bar Sheshet Parfat (1326-1408) Responsa Section 157:

The kabbalists pray once to this sefirah and then to another sefirah, as the prayer requires.... And that is very strange to a person who is not a kabbalist like they are, and they [the non-kabbalists] think that this is belief in a multiplicity [of Gods]. I have heard a philosopher speak in a defaming manner of the kabbalists, and he used to say: "The Gentiles [Christians] are believers in a trinity, and the kabbalists are believers in a ten-ity."
I once asked the old wise Don Joseph ibn Shushan, may he rest in peace,... "How do you kabbalists direct yourselves in one blessing to a certain sefirah, and in another blessing to a different sefirah? Furthermore, do the sefirot have divinity that a person should pray to them? And he answered me: God forbid that the prayer should be directed to anything other than to the Holy One, the cause of all causes. The thought [in prayer] is directed only to drawing down the abundance from above from the sefirah relevant to what is being prayed for."

Rabbi Moshe Haim Luzzato (1707-1746), Sefer Kinat Adonai Zivaot, Part 1.

The enemies of the Wisdom of Truth have called the believers in it, "Believers in ten-ity." ... They have said, "Anybody who believes in Divinity other than His, is but a denier of the God of Israel."
And we answer them by saying that the sefirot are not separate from the one who emanates, for they are like the flame connected to the coal, and all is one, a unity that has within it no division.

Translated by Jerome I. Gellman, Ben Gurion University of the Negev