Answer:

There is a sense in which this is a trick question, although I am sure you had no intention of posing a trick question.  It reminds me of the situation in which Jesus was asked about whether one should pay taxes to Caesar.  Simply saying either yes or no would have gotten Jesus in trouble.

The reason I say this is that, depending on what you mean by the question, the answer is either yes or no.

The simple answer, of course–one which can be supported by an innumerable list of Bible verses–is that the grace of God is unconditional.  In other words, as we can read in Ephesians 2:8-10, the grace of God is not conditional upon any particular works which we do.  Across the page, we find in Ephesians 1:13-14 that the Holy Spirit one receives upon salvation "is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession."  The opportunity for salvation is already out there.  Jesus already paid the full price.  There are no "conditions."  There is no fine print.  There is no single sin which, if we committed it, would steal our salvation.  There is no single act we must perform after being saved which, if we fail to do it, would rob us of the grace of God.

So, the simple and I believe the most fundamental answer is that the grace of God is unconditional.

However, one can give the opposite answer, depending on exactly what is meant by the question.  First of all, there is the receiving of the grace of God.  Obviously, not all people are saved.  Therefore, almost by definition, the grace of God is "conditional."  One condition of the grace of God in receiving it is that we must have faith in God.  We must put the full weight of our trust in the saving effect of the blood of Jesus.  We must repent and be baptized in order to receive the grace of God.  This is seen in Acts chapter two.  Men who were "cut to the heart" by the message of salvation through Jesus asked "what shall we do?"  Peter did not respond by saying "Oh, nothing.  You are already saved.  You already have the grace of God."  No he told them to repent and be baptized in order to receive the grace of God, which is the forgiveness of sins and the gift of the Holy Spirit.

In this sense, the original receipt of the grace of God is conditional.

And what about those who have received the gift of grace?  Is it, at this point, unconditional to them?  Of course  the answer is yes, as mentioned above.  Yet, it is possible for one to lose their salvation.  Although it is true that perfect obedience is not required.  Although it is true that no single sin or single act of disobedience can cause us to forfeit the grace of God, it is possible for a person through neglect and through willfully continuing in sin (Hebrews 10:26-31) to lose salvation.  This is the whole point of the book of Hebrews.  This is the reason for such warnings as Hebrews 6:11-12 and Hebrews 3:12-4:5.  The grace of God is not conditional, but if the saved person chooses to turn his or her back on God and no longer follow the path of salvation, God will accept this decision and not accept such a person into heaven.  In this sense, one can say that the grace of God is conditional upon the recipient continuing to walk in the grace of God and to show concern for being saved.

So, the answer is yes and no.