Why are so many believers turning away from faith according
to the scriptures?

Jesus explained it in His famous "last things" teachings and Matthew
recorded it in Matthew 24 {also Luke in Luke 13): (1) because sin will
abound, and (2) because of this increase in sin and lawlessness, the love
of many Christians will wax cold.

"Then many false prophets will rise up and deceive many {the
deceived seldom know it is happening}
.  And because lawlessness
will abound the love of many will wax cold.  But he who endures to
the end, shall be saved."  (Matthew 24:11-13)

So one might say easily based on Jesus and the Bible that the rapid
increase among churches and Christians due to the turning away from
faith is primarily because of:  (1) the work and proliferation of many false
prophets, which is working in that many are deceived and led away from
truth into half truth, lies, and tangents; (2) the rising tide of the moral decay
of sin and lawlessness in the world that Paul described in I Thessalonians
as the "falling away" has that vast increase as the Holy Spirit is withdrawn
from the world, however we can not blame Satan for the ever present
"spirit of sin within us that lusteth to envy and covet"  (James 4:5); and (3)
the lukewarmness of Christians and church members has been a major
problem since the days the Risen Christ through the Apostle John writing
the book of Revealtion criticized church membetrs for being "neither hot
or cold", however what is more influential in these current times is the bad
results coming from the 5 stages of the Falling Away of II Timothy
4:4,5--(a) an intolerance among Christians of sound doctrine, generally
disguised as keeping up with a new generation or a new age, (b)
Christians more motivated by purpose driven personal wants and desire
than the Spirit of Christ {this is more of sin abouding and the love of
Christ, Bible, and God waxing cold as compared to the "cares of this
world, and the deceitfulness of riches", (c) preferential treatment by pulpit
committess and members for "itching ears teachers" {it is always the
other guy or guys and not us or our church}, (d) as church members turn
away from truth, they naturally turn away from Christ and toward more
habitual sin {"Beware brethren," Paul wrote and it is recorded in the
scripture of Hebrews 3:1, "that there be in any of you an evil heart of
unbelief in departing from the living God"}, and (e) in the last and final
stage of the historical great apostasy predicted in Scripture, the being
"turned into fables" because of the impact on church members has
become so influential that they are blinded from the reality of their
blindness.  In other words, they no longer have "eyes to see and ears to
hear".