From here:
“The Tau was the basis for what is now called the “cross” taken from the Latin “crux”. “The shape of the [two-beamed cross] had its origin in ancient Chaldea, and was used as the symbol of the god Tammuz (being in the shape of the mystic Tau, the initial of his name) in that country and in adjacent lands, including Egypt. By the middle of the 3rd cent. A.D. the churches had either departed from, or had travestied, certain doctrines of the Christian faith. In order to increase the prestige of the apostate ecclesiastical system pagans were received into the churches apart from regeneration by faith, and were permitted largely to retain their pagan signs and symbols. Hence the Tau or T, in its most frequent form, with the cross-piece lowered, was adopted to stand for the cross of Christ.”—An Expository Dictionary of New Testament Words (London, 1962), W. E. Vine, p. 256.
“It is strange, yet unquestionably a fact, that in ages long before the birth of Christ, and since then in lands untouched by the teaching of the Church, the Cross has been used as a sacred symbol. . . . The Greek Bacchus, the Tyrian Tammuz, the Chaldean Bel, and the Norse Odin, were all symbolized to their votaries by a cruciform device.”—The Cross in Ritual, Architecture, and Art (London, 1900), G. S. Tyack, p. 1.
The people of the ancient lands used the cross in worship, some, like the Egyptians used it in Phallus worship, or, worship of the male sex organ. It was used as a symbol of fertility. “Various figures of crosses are found everywhere on Egyptian monuments and tombs, and are considered by many authorities as symbolical either of the phallus [a representation of the male sex organ] or of coition. . . . In Egyptian tombs the crux ansata [cross with a circle or handle on top] is found side by side with the phallus.”—A Short History of Sex-Worship (London, 1940), H. Cutner, pp. 16, 17; see also The Non-Christian Cross, p. 183.
The Ancient Church by clergyman W. D. Killen says, on page 316: “From the most remote antiquity the cross was venerated in Egypt and Syria; it was held in equal honour by the Buddhists of the East; and, what is still more extraordinary, when the Spaniards first visited America, the well-known sign was found among the objects of worship in the idol temples of Anahuac. It is also remarkable that, about the commencement of our era, the pagans were wont to make the sign of a cross upon the forehead in the celebration of some of their sacred mysteries.” The origin of the cross is indeed very pagan.
“The Greek term signifies an upright stake on which criminal were executed, with no suggestion of a cross-beam. In the Latin versions the term ‘crux’ was used, but according to Livy of the 1st century B.C., the word meant no more than an upright stake; it was only later that crux came to mean a cross. Josephus relates how 2,000 were crucified at one time (‘Antiquities’ book 17; 10:10) hardly practicable if crosses had to be made for each one. There are Greek words which denote a cross, but none of these appear in the in any of the four gospel accounts of Jesus’ execution. At Galatians 3:13 Paul refers to the instrument as ‘a timber’ (A.V. a tree) a reference to the upright stake on which bodies of criminals were hanged under the Mosaic Law (Deut 21:22), and which Jesus fulfilled by his death.
Some have contended that the Romans did use crosses for execution at that time although Livy refutes this. Even if this were so, the Romans were also careful to observe local customs as fas as possible to avoid unnecessarily upsetting the populace, and so likely would have modified their method to conform to the Jewish practice. A rough upright stake would be in any case less trouble to produce than a hewn cross with a joint strong enough to bear the weight of a man.
Christians are sometimes disturbed to learn that the cross, considered for centuries as a Christian symbol, had its origin long before Christ and was actually used in pagan mythology.It was the symbol of the god Tammuz, and Bacchus, and the Egyptian Osiris. It was worshipped by the Celtic druids and worn on necklaces by the Vestal Virgins of Rome…As the Greek text shows, Christ was not executed on a Cross, that symbol can be regarded for what it is, a pagan corruption of Christian worship introduced in the early centuries of our common era. Thus in harmony with 2 Cor 6:15 although long cherished, it is something that Christians should shun.”