Yes but so are Muslims and Orthodox Jews. God has built in a moral code in all of us. Some just suppress it more than others.
However, the spiritual decline is also overall seen across the board in all "Christian" faiths
There have been periods throughout history of spiritual declines and then great awakenings. Why God is withdrawing His hand of grace can only be seen as His way of calling His people though I suspect we may be approaching the return of Christ. Beyond that it is a puzzlement.
This relates to the dichotomy btwn God's sovereignty and mans accountability. Nothing can thwart what He has determined, or serve Him apart from His grace and enablement, yet He holds man accountable for not doing what they were commanded to do, and thus could have done by Him.
It is not a matter of "God will save the heathen when He is good and ready, so you need not do anything," as some hyper Cs may say, but that God has determined that our response effects others for good or for bad, yet it all works toward the fulfillment of God's plan.
"And I sought for a man among them, that should make up the hedge, and stand in the gap before me for the land, that I should not destroy it: but I found none. Therefore have I poured out mine indignation upon them; I have consumed them with the fire of my wrath: their own way have I recompensed upon their heads, saith the Lord God. " (Ezekiel 22:30-31)
"And herein is that saying true, One soweth, and another reapeth. I sent you to reap that whereon ye bestowed no labour: other men laboured, and ye are entered into their labours. " (John 4:37-38)
"And we know that all things work together for good to them that love God, to them who are the called according to his purpose. " (Romans 8:28)
How He all does that is a mystery.
"O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out! " (Romans 11:33)