The question is posed, “How do we know that we have the Spirit of God abiding within us”? Can we know for sure?
Romans 8:16
The Spirit itself beareth witness with our spirit, that we are the children of God:
“Bear witness” is defined as “providing evidence for…”
http://dictionary.reference.com/browse/bear%20witness
So the Spirit itself is to be our evidence. But how do we know if the Spirit is abiding within us?
Romans 8:
9But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
10And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.
11But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
We all have an “inner voice” that most often is referred to as our “conscience”. It is this “conscience” that would guide all men in their actions. Some men do what they consider to be right and proper; others do what they think they can get away with. Some don’t even know the difference! In those that are dead in sin, this voice says to live after the flesh, doing the things of the flesh, or whatever our heart desires. In Christians, this voice speaks to us in relation to the Word of God, telling us that our actions must conform to God’s standard of conduct as found in the Word.
It is the work of the Spirit by the blood of Christ to purge this conscience from dead works to serve the Living God (Heb 9:14). When the Christian is converted, he is granted fellowship with the Spirit of God by the blood of Christ and the Comforter is come to lead and guide into all truth.
John14:26
But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
Isaiah30:21
And thine ears shall hear a word behind thee, saying, This is the way, walk ye in it, when ye turn to the right hand, and when ye turn to the left.
In the OT, God referred to the renewing of our conscience as the two “hearts”:
Ezekiel 36:
26A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.
27And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
God promised us that he would remove the “stony heart” and give us a heart of “flesh” or one that is tender to the voice of His Spirit, causing us to “walk” in His “statutes”.
Sinners don’t have this fellowship of the Spirit of God due to their “stony” heart, thus they seek not the truth and naturally do the works of the flesh.
John 14:17Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
Rom 8:13For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
The Christian on the other hand, manifests the works of the Spirit for in the Spirit he is “alive unto God through Jesus Christ our Lord”. Romans 6:11
Gal 5:
22But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
23Meekness, temperance: against such there is no law.
24And they that are Christ's have crucified the flesh with the affections and lusts.
25If we live in the Spirit, let us also walk in the Spirit.
The sinner manifests the works of the flesh.
Galatians 5:
17For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
18But if ye be led of the Spirit, ye are not under the law
19Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
20Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
21Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
I encourage all to seek out the definitions of these works from the dictionary; their meanings might be a surprise.
Finally, to make certain it is the Spirit bearing witness with our conscience; we are admonished to study the Word:
2Timothy2:15
Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.
If our conscience and our actions bear witness with the Word, we can rest assured:
John 14:
23Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.
24He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father's which sent me.
1 John 2:
5But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.