Question: Why do you believe that "by His stripes we were healed" only refers to healing from sin, ...

Question: Contrary to what you’ve said, “By His stripes ye were healed” does refer to physical healing. Matthew:8:16-17 [16] When the even was come, they brought unto him many that were possessed with devils: and he cast out the spirits with his word, and healed all that were sick: [17] That it might be fulfilled which was spoken by Esaias the prophet, saying, Himself took our infirmities, and bare our sicknesses.
See All... quotes Isaiah:53:4Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted.
See All... and does indeed show that it is physical. I pray our LORD will show you this truth. God warns of a form of godliness, denying its power.

Response: I believe in miracles and have seen God do many. God still heals in answer to prayer. I have been instantly healed myself and have seen others, for whom I prayed, instantly healed.

The Bible, however, does not promise anyone physical healing from sickness in this life “by His stripes” in the same way that it promises spiritual healing from sin. I can assure anyone who will believe the gospel that he or she will be instantly saved from the eternal penalty of sin and receive eternal life as a free gift of God’s grace. I cannot give anyone the same assurance of being perpetually healed from physical ailments. Can you?

If we have physical healing through the cross of Christ in the same way we have spiritual healing, then Christians ought to live much longer than non-Christians and there ought to be at least some Christians who have lived 100, 200, 300 or more years. But this is not the case. Our souls and spirits have been redeemed and as proof we are indwelt by the Holy Spirit (Ephesians:1:13-14 [13] In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, [14] Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
See All...), but we together with “the whole creation groan and travail…[until the redemption of our body (Romans:8:18-25 [18] For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us. [19] For the earnest expectation of the creature waiteth for the manifestation of the sons of God. [20] For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope, [21] Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God. [22] For we know that the whole creation groaneth and travaileth in pain together until now. [23] And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body. [24] For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for? [25] But if we hope for that we see not, then do we with patience wait for it.
See All...).

You correctly say that Matthew:8:16-17 [16] When the even was come, they brought unto him many that were possessed with devils: and he cast out the spirits with his word, and healed all that were sick: [17] That it might be fulfilled which was spoken by Esaias the prophet, saying, Himself took our infirmities, and bare our sicknesses.
See All..., when it speaks of the healings by Jesus before He went to the cross, quotes Isaiah:53:4Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted.
See All... (“he that hath borne our griefs [sicknesses] and carried our sorrows [pains]”). It does not quote Isaiah:53:5But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed.
See All... (“by his stripes we are healed”). That verse, which deals with sin (“wounded for our transgressions, bruised for our iniquities”), is quoted in 1 Peter:2:24Who his own self bare our sins in his own body on the tree, that we, being dead to sins, should live unto righteousness: by whose stripes ye were healed.
See All... (“bare our sins…by whose stripes ye were healed”). Isaiah:53:4Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted.
See All... was fulfilled by Christ’s healing ministry: Isaiah:53:5But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed.
See All... was fulfilled at the Cross.

As for “denying the power thereof,” Paul reminds us that the gospel “is the power of God unto salvation” (Romans:1:16For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
See All...). I do not deny the power of God in any way today. I simply oppose those who profess the “power of the Spirit,” claim healings that don’t occur, and destroy the faith of many with unbiblical promises of healing, which bring guilt when not realized.