His treatise is a letter he sent to Christians in the city of Rome and it is in the eighth chapter that he poses the question and notes that no external influence can cause separation.
Yet Paul, the Apostle, knew the whole answer which began in Moses' book of Genesis, in Chapter 3 when God asks of His creation "where are you?", Adam was afraid for he had disobeyed the Lord God and hid himself. The answer was finalized by the visionary prophet Isaiah when God used his pen to say: "Your iniquities have separated you from your God; and your sins have hidden His face from you, so that He will not hear" (Isa 59:2.)
The question remains: who can separate us from the love of God? And the answer is — we can when we choose not to follow his ways. We can when we disobey his rules for living. We can when we hide behind our habitual sin. We can when we decide to be unholy and unworthy.
But, if we choose to allow Jesus to redeem us, save us, make us Holy, affirm that we are worthy — Then NOTHING can separate us from the love of God which is in Christ Jesus our Lord!