contestante Deo

After it was at first spoken through the Lord, it was confirmed to us by those who heard, God also testifying with them, both by signs and wonders and by various miracles and by gifts of the Holy Spirit according to His own will. (Hebrews 2:3-4)

Having exhorted the reader to give heed to what God has spoken through Jesus, and having reminded us of the utter futility of returning to "what was spoken through angels," the writer confirms the efficacy and substance of the salvation that Jesus brings. Here, in verses 3-4, the author proceeds upon this thought and reaffirms to the reader both the validity and authenticity of the great salvation referenced beforehand. This supreme salvation originates with no less than the Person of Jesus Christ, and it was faithfully communicated by those followers who knew and walked with Him. And, while God entrusted this glorious message to the disciples of Christ so that they might begin to proclaim His gospel to the ends of the earth as Jesus commanded (see Matthew 28:19-20), God did not (and does not) stand afar in the process: contestante Deo, "God also testif[ied] with them" (Hebrews 2:4, Vulgate; 2:4, NASB).
If we stop and think about it, the statement in 2:3-4 is an unabashed admittance that what is proclaimed by the author is second-hand information. Yet, rather than weakening the force of the argument, the writer portrays God's faithfulness in being actively involved in the communication of the gospel as it is transmitted from generation to generation. When we approach this passage, we must take care not to import the present tendency (especially prevalent in Western culture) to disdain oral history, assuming that it inherently lacks precision and legitimacy. Hebrews reminds us that, prior to the production and compilation of written texts, the apostles of Christ faithfully transmitted Jesus' words to subsequent generations. Moreover, if we are still in need of verification, what greater proof can we find than God Himself bearing witness to the words of Jesus' followers with "signs and wonders and and by various miracles and by gifts of the Holy Spirit according to His own will" (2:4)?
Not just for the early Christians who knew the apostles, but for our sake, as well, these events have been passed down, and many of them are recorded in Scripture, for example throughout the book of Acts. Turning there, we encounter case after case after case of God bearing witness to the validity of Christ's life, death, and resurrection through manifold signs, wonders, and miracles "at the hands of the apostles" (here I merely provide a brief citation for some instances, but I encourage you to read each of these passages, along with what they refer to, in their entirety):

Everyone kept feeling a sense of awe; and many wonders and signs were taking place at the hands of the apostles. (Acts 2:43)

At the hands of the aposltes many signs and wonders were taking place among the people. (Acts 5:12)

And Stephen, full of grace and power, was performing great wonders and signs among the people. (Acts 6:8)

Therefore they spent a long time there speaking boldly with reliance upon the Lord, who was testifying to the word of His grace, granting that signs and wonders be done by their hands. (Acts 14:3)

The book of the Acts of the Apostles is replete with records of speaking in tongues and diverse languages, healings, casting out of demons, and raising people from the dead. When I read these accounts, I cannot help but see manifest the love of God toward us in that He meets us in our needs (misguided as they may be). For, all too often, we are of the mindset that, unless we see signs and wonders, we will not believe (see John 4:48). But, just as Jesus lovingly admonished Thomas: "Blessed are they who did not see, and yet believed" (John 20:29).
This latter truth from Jesus holds for us today just as it held for those to whom Hebrews was written. The distance between us and Jesus (that is, from now until when He spoke on earth) is diminished by the faithful and accurate transmission from generation to generation of the words that Jesus spoke. For this reason, we have no fear in giving ourselves wholly to the words of Jesus Christ and the great salvation that is found in His words of life (see John 6:63). As Peter profoundly declared, "Lord, to whom shall we go? You have words of eternal life" (John 6:68). The author of Hebrews ensures the reader that what is entrusted to them is the same as what the apostles preached, with God testifying with them, and it is the same as what Jesus Himself declared "in the days of His flesh." The message of the gospel originates from Christ and declares Christ as its substance. What the apostles taught, and what Hebrews proclaims, is no fabricated story - these people were moved forever by the Person and work of the divine Christ, they were motivated by the reality of Christ and the necessity (compelled by love) of communicating that reality to all of humankind for the glory of God and for our salvation.

Great is the LORD, and highly to be praised, and His greatness is unsearchable. One generation shall praise Your works to another, and shall declare Your mighty acts. On the glorious splendor of Your majesty and on Your wonderful works, I will meditate. Men shall speak of the power of Your awesome acts, and I will tell of Your greatness. They shall eagerly utter the memory of Your abundant goodness and will shout joyfully of Your righteousness. (Psalm 145:3-7)