The Greek preposition phrase is εἰς ἡμᾶς (eis hēmas). The preposition εἰς preceding an accusative noun is frequently translated as "in" or "to," so both are grammatically possible.
Here's a list of all the verses in which this prepositional phrase occurs. The majority of them are Pauline epistles.
- Acts 3:4 "look at us" (or "toward us")
- Romans 5:8 "showed His love to us"
- Romans 8:18 (translation in question)
- Romans 16:6 "labor unto us" (or "to us" or "toward us")
- 2 Corinthians 1:5 "sufferings abound in us"
- 2 Corinthians 1:11 "gift to us"
- Ephesians 1:8 "abounded toward us" (or "to us")
- Ephesians 1:19 "toward us" (or "to us")
- 1 Thessalonians 4:8 "given to us"
- Hebrews 2:3 "to us"
- 1 Peter 1:4 "for us" (KJV actually mistranslates the pronoun as "you")
- 2 Peter 3:9 "toward us" (or "to us")
The only real solution is examination of the context. Here are some important phrases to consider.
Paulos writes that "He who raised Christ from the dead shall also vivify your mortal bodies by His Spirit who dwells in you" (Rom. 8:11). When we are resurrected from the dead unto eternal life, we shall be raised with a glorious (glorified) body (Phil. 3:21). Furthermore, he writes, "Because the creation itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God (Rom. 8:21). Again, there appears to be another allusion to the resurrection of the dead.
While it is true that Christians are considered as adopted into God's family as soon as they are born again, and therefore are "children of God," Paulos clarifies what he means when he refers to "children of God" when he says, "...even we ourselves groan within ourselves, waiting for the adoption, the redemption of our body" (Rom. 8:23). In this particular context, a child of God is one who is redeemed in every sense, spiritually and physically --- physically, by the resurrection in an immortal, incorruptible, glorified, spiritual body.
Furthermore, he says that we are predestined (Rom. 8:29) "to be conformed to the image of his Son, so that he may be be the firstborn among many brethren." The context here is the resurrection of the dead. As Christ was raised in an incorruptible body, first before all others, he was the firstborn among many brethren, and we shall all be conformed to his image, possessing immortality and incorruptibility in a spiritual body.
I believe the central theme is that man will be liberated from the bondage of corruption and mortality by the resurrection of the dead when he will possess an immortal, incorruptible, glorified, spiritual body. Therefore, although there will be glory revealed to us in many ways, e.g., when we dwell in heaven, and when we see the Son and the Father, in this particular context, Paulos is referring to the glory which will be revealed in us by our resurrection from the dead in a glorified body.
And those who shall be wise shall shine as the brightness of the firmament, and those who turn many to righteousness as the stars, forever and ever. (Dan. 12:3)