Satan is capable only of temptation .... It is man free will to choose to do evil.... and side with the evil one.
Evil existed in the world due to the "fallen angel" of God who out of his choice .... abandoned his calling to be with God and istead became (out of pride) evil...as God is not capable of creating anything evil...
As far as an explanation on "why so many religions"?
IMO we are all created equally in the image and likness of God. God has revealed himself in history .... Lots of people have revelations i.e. Babylonians .... etc. God worked through the people of Israel to reveal Himself, in the Old Testament and in the New Testament. Christianity came forth.... Chirst did say that the end of the world will come when all people will be evangelized... Since He did not speak about "the other religions" I do not think it is a matter that endangers our salvation... to say that we cannot judge others...who believe otherwise... I do not say that we should tolerate percecution against Christians.....though. IMO other religious will be judged according to their own laws and God's judgment. And... wheather they are "created by satan" yes I do believe that some who do believe they are "satanic" and have to do with satanism.... they must be his creation.... since PEOPLE who believe in satan they formed them..... I do believe that evil exists and is active in this world and people have become vessels of their dreadful master... allowing themselves to become his slaves....
It is ridiculous to think that Christians only see satanic the other faiths ..... and pretty imature IMO also a great cover up for "accusing" the Christians as been "discriminatory" against them.... I do not know about other denominations but EO in general do not feel that other religions are necessarily "satanic"
Revelation through God's glory, even though the mystery is "beyond"
Our exploration of an Orthodox attitude toward non-Christian religions begins with the Christian understanding of God. Emphasis is on the mystery of divine reality - the essence of God - which exceeds human capabilities. It is a basic truth of Orthodox Christianity that God's essence is incomprehensible and inaccessible to the human person; it is "beyond" all creaturely approach. A prayer in the Divine Liturgy expresses it as follows: "... for you are God ineffable, beyond comprehension, invisible, beyond understanding, existing forever and always the same ... "
[2] A minor change in the rendition emphasizes the fundamental truth. God's essence is totally "beyond" - "beyond verbalization, beyond comprehension, beyond vision, beyond understanding."
Yet, while the essence of God is beyond communion, God reveals Himself through His Glory. The human person participates in God's energies manifested as theophanies "The glory of the Triune God embraces the universe
(ta pania) and brings all things within the scope of His love."
[3] God's glory
(doxa, kaboth, shekhina) is revealed to human persons in their true intimate relation as an, end and fulfillment of the original creation of man.
The revealed glory of God - his energies - penetrates all creation and is the starting point for Christian life and hope. This central truth of Christianity was communicated doxologically to Isaiah (6:3), and is articulated in the angelic hymn of the Divine Liturgy which accompanies the prayer noted above: "Holy, Holy, Holy are You the Lord of Hosts, heaven and earth are filled with Your glory." This hymn, on the one hand, expresses the total mystery of God and, on the other, notes that His divine glory and love encompass all forms of life, His entire creation.
[4]
Also:
Truth and Tolerance
As has been emphasized, the issue of Christian Truth is of highest importance in the Orthodox view of other religions. Pontius Pilate asked "What is Truth?" (John 18:38). He posed this question to Jesus who standing before him, remained silent. Christians interpret this silence as His reply that the Truth was standing before him - Christ is the Truth.
The Byzantine Empire identified itself as an Orthodox Christian state, however, it allowed for diversity of religious practices within its borders. "In Byzantium, the recognition of Christianity first as a privileged religion, and then as the official religion of the Empire, did not affect the basic principle of tolerance toward the members of other religions. But it restricted the rights they were permitted in public life. Christianity and, after the East-West schism (1054), Orthodoxy were closely linked to the identity of the Byzantine state and thus determined its religious policies."
[31]
For Orthodoxy there is a fusion between the truth claim of Christianity and a mandate for tolerance. We may say that one can not be a Christian without embracing tolerance as a concomitant of Christian love. This most significant and long-standing teaching of tolerance in Orthodoxy is emphasized in an encyclical letter of Ecumenical Patriarch Metrophanes III (1520-1580). This document was written to the Greek Orthodox in Crete (1568) following reports that Jews were being mistreated. The Patriarch states, "Injustice ... regardless to whomever acted upon or performed against, is still injustice. The unjust person is never relieved of the responsibility of these acts under the pretext that the injustice is done against a heterodox and not to a believer. As our Lord Jesus Christ in the Gospels said do not oppress or accuse anyone falsely; do not make any distinction or give room to the believers to injure those of another belief."
[32]
Today many Orthodox Christians live in societies of cultural, linguistic and religious pluralism. This has cultivated and nourished a deeply-felt attitude of respect, tolerance and understanding toward other people and their religions. The Orthodox Church has no official pronouncement on this matter. However, the long-standing tradition of respect and tolerance for other faiths is well stated by Archbishop Anastasios: "Being created in the image of God, every human being is our brother and sister."
[33]
That is from my prof. of Cults and World Religions.... here is the link:
http://www.goarch.org/en/ourfaith/articles/article8089.asp
Hope that sheds some light....
God bless,
Philothei
(the answer's been much better explained below)
I believe that other religions are "incomplete" as others here have said, and are not necessarily from satan....they're just human attempts to get to know God, whereas Christianity is God coming down to meet us. However, I do believe that the only way to the Father is through the Son, so if someone feels lead to another religion that feeling might be from the enemy. 
