List of old heresies condemned by the Catholic Church now present in Mormonism and Jehovah's Witnesses

The Church of Jesus Christ of Latter Day Saints (Mormons) fall foul of the basic Christian tenants of belief with regard to the Trinity and to the nature of Christ. The Apostles' Creed and Nicene Creeds form the basis for Christian faith and any denomination that departs from those are condemned by the Catholic Church as heretical.

"There are three separate persons in the Godhead: God, the Eternal Father; His Son, Jesus Christ; and the Holy Ghost. We believe in each of Them (A of F 1:1). From latter-day revelation we learn that the Father and the Son have tangible bodies of flesh and bone and that the Holy Ghost is a personage of spirit, without flesh and bone (D&C 130:22–23). These three persons are one in perfect unity and harmony of purpose and doctrine (John 17:21–23; 2 Ne. 31:21; 3 Ne. 11:27, 36). It is generally the Father, or Elohim, who is referred to by the title God. He is called the Father because He is the father of our spirits (Num. 16:22; 27:16; Mal. 2:10; Matt. 6:9; Eph. 4:6; Heb. 12:9)."

"The God known as Jehovah is the Son, Jesus Christ (Isa. 12:2; 43:11; 49:26; 1 Cor. 10:1–4; 1 Tim. 1:1; Rev. 1:8; 2 Ne. 22:2). Jesus works under the direction of the Father and is in complete harmony with Him. All mankind are His brothers and sisters, for He is the eldest of the spirit children of Elohim."

"The Holy Ghost is also a God and is called the Holy Spirit, the Spirit, and the Spirit of God, among other similar names and titles. With the aid of the Holy Ghost, man can know the will of God the Father and know that Jesus is the Christ (1 Cor. 12:3)."

Source: https://www.lds.org/scriptures/gs/god-godhead?lang=eng&letter=g

Three Gods who are separate and distinct beings. Their Jesus is not the eternal and uncreated Word of God who was with God in the beginning and who is God.

Mormons belive "As man now is God once was. As God now is, man may be."

They also believe in baptisms for the dead.

And they reject the authority of the Pope.

It would take too long to list all the pre-Reformation heresies that could be applied to Latter Day Saints, but is there any need to go further than the fact they reject the Christian definition of the Trinity and who Jesus Christ really is?

The link given by depperm will give you all the necessary details on pre-Reformation heresies condemned by the Catholic Church. See specifically: https://en.wikipedia.org/wiki/List_of_Christian_heresies#Trinitarian/Christological_heresies

All of this from Wikipedia, and simplified:

Arianism was the view held by some 4th century christians that God the Father, Jesus Christ, and the Holy Ghost were three seperate and different essences (beings), and that the later two derived their godhood from the Father, were created and so on. They were seen as inferior to God.

Semi-Arianism differed from Arianism in that it viewed God and Jesus Christ as seperate, but similar essences. As such it places Jesus Christ on the same level as God. The Holy Ghost is seen as different.

Both views were condemned by the Nicean creed which makes the view "same substance/essence" explicit. Other answers go into more detail here.

LDS theology can't really be said to be Arian or Semi-Arian, but shares the view that God the Father, Jesus Christ and the Holy Ghost are three distinct beings. Since all spirits are both co-eternal with God, and made into his children at some point, one could say LDS are somewhere in-between both views.

I can't say much about Jehovahs Witnesses, but I assume the doctrine can also be (very roughly again) compared to Arianism and will fall somewhere completely different on the spectrum than LDS doctrine.

Audianism (also 4th century) taught that God literally created mankind in his image. LDS teach this too, including the view that God has a physical body.

In 325 A.D. the Church produced the Creed of Nicaea (to distinguish it from the Nicene Creed) in order to exclude one particular heresy doing the rounds back then. Examination of the Creed of Nicaea shows both the Latter Day Saints and Jehovah’s Witness religions supporting that heresy, in different ways, and to different degrees. I do not intend to do a comparison between the two modern groups (you only ask for a list), simply to state the Creed of Nicaea so that those familiar with LDS and JW doctrines on the Person of Christ will see how they fall foul of the heresy spelled out in that ancient document. I do not think Stack Exchange accepts simple lists, but requires evidence to support answers, hence the details I provide. Because the opening sentence runs on (and on, and on) I quote the first four paragraphs before picking out the various heresies each modern group supports. Here is what the Creed of Nicaea says:

“We believe in one God, the Father, Almighty, maker of all things visible and invisible;

And in one Lord Jesus Christ, the Son of God, begotten of the Father, only-begotten, that is, from the substance (ousia) of the Father; God from God, Light from Light, Very God from Very God, begotten not made, of one substance (h*m*ousios, consubstantial) with the Father, through whom all things were made, both in heaven and on earth; who for us men and for our salvation came down and was incarnate, was made man, suffered, and rose again on the third day, ascended into heaven, and is coming to judge the living and the dead;

And in the Holy Spirit.

And those who say: ‘There was a time when he was not’, and: ‘Before he was begotten he was not’, and: ‘He came into being from nothing’, or those who pretend that the Son of God is ‘Of another substance (hypostasis), or essence (ousia) [than the Father] or ‘created’ or ‘alterable’ or ‘mutable’, the catholic and apostolic church places under a curse.”

Please note how this ancient document is penned by “the catholic and apostolic church” and how they state that those disagreeing with this Creed are anathema – cursed. That is because they are viewed as supporting the heresy the Creed was designed to expose.

Now, the LDS are anathema (heretical) due to claiming there are many Gods/gods. They teach that the god Elohim fathered his firstborn via a spirit wife, and he was called Jehovah (who had, as a younger brother, Lucifer). This Jehovah became a god himself, and also became the man, Jesus Christ. They teach that there was a time when the Son of God ‘was not’. Hence the LDS violate the first sentence plus paragraph four. That amounts to two particular heresies.

The JWs are anathema (heretical) due to claiming that Jesus was created by Jehovah as the archangel Michael. The JWs disagree on the meaning of ‘only-begotten’, saying that it means he was the first-created creature of Jehovah God, whereas the Creed stresses that ‘begotten’ means the opposite of ‘made’. They are cursed because of disagreeing with paragraph four of the Creed; one particular heresy. They also claim Jesus is a different 'god' to Jehovah God, being 'a god' as per their rendition of John 1:1, which likewise makes them polytheistic, as are the LDS. That is a second heresy.

Please note, this answer does not claim to be an exhaustive list. But it should suffice to give a categoric answer to the question as to why LDS and JW denominations are viewed as heretical by Catholicism (and Protestantism also, let it be noted) today.

Source: Article ‘What Really Happened at Nicea?’ By James R. White, Christian Research Journal, Spring 1997 pp 30-34