Here is a helpful extract from Berkhof in defense of common grace and the free offer:
It is sometimes argued that the Reformed doctrine of common grace [and it is a Reformed doctrine] involves the doctrine of universal atonement, and therefore leads into the Arminian camp. But there is no good ground for this assertion. It neither says nor implies that it is the purpose of God to save all men through the atoning blood of Jesus Christ. The objection is based particularly on the universal proclamation of the gospel, which is considered possible only on the basis of a universal atonement. It was already suggested by the Arminians at the time of the Synod of Dort, when they asserted that the Reformed with their doctrine of particular redemption could not preach the gospel to all men indiscriminately. But the Synod of Dort did not recognise the implied contradiction … and this is in perfect harmony with Scripture, which teaches on the one hand, that Christ atoned only for the elect, John 10:15; Acts 20:28; Rom. 8:32,33; cf. also John 17:9; and on the other hand, that the gospel call must be extended to all men indiscriminately, Matt. 22:2-14; 28:19; Mark 16:15,16. If it be objected that we cannot fully harmonise the indiscriminate and sincere offer of salvation on condition of faith and repentance with the doctrine of particular atonement, this may be admitted but with the distinct understanding that the truth of a doctrine does not depend on our ability to harmonise it with every other doctrine of Scripture.
Louis Berkhof, Systematic Theology, 444-5