GOD ALSO TESTIFYING WITH THEM: sunepimarturountos (PAPMSG) tou theou. (Mark 16:20; John 15:26; Acts 2:32,33; 3:15,16; 4:10; 14:3; 19:11,12; Romans 15:18,19)

Regarding the statement God also testifying, Spurgeon observes that...

Those who doubt the truth of the gospel, or who say they do, are often found believing historical statements that are not half as well proved. A man sits down, and reads the book of the Gallic wars, and he believes that Julius Caesar wrote it; yet there is not a half or a tenth as much evidence to prove that he did write it as there is to prove that our Lord Jesus lived, and died, and rose again from the dead. The witness to the truth of these great matters of fact has been borne by God himself with signs, and wonders, and miracles. Honest and true men, apostles and others, have witnessed to them; and they have also been certified by Incarnate Deity, even by the Lord who deice to speak to us by his Spirit. We cannot, therefore, trifle with this gospel without incurring most serious guilt.

Observe, then, that this gospel comes to us by Christ, and it is confirmed to us by His apostles, and further confirmed by those signs and wonders, and divers miracles, which God sent as the seals of apostolic teaching; so that this spell is not one about which we can raise any question whatever. It comes by a medium which we must not dare to question, it has confirming seals in it which it is blasphemous for us to dispute. Oh, how gladly should we receive it! How tenderly should we treat it? How devoutly grateful should we be for it; and how earnestly should we comply with all its requirements?

This gospel of ours is stamped with the seal of God; He has set His mark upon it, to attest its genuineness and authority. The miraculous gifts of the Holy Spirit were the seal that the gospel was no invention of man, but that it was indeed the message of God. Gifts of healing, gifts of tongues, gifts of miracles of divers kinds, were God’s solemn declaration to man, “This is the gospel; this is My gospel which I send to you; therefore, refuse it not.”

Bearing witness with them (4901) (sunepimartureo from sun = with, speaks of intimate relation + martureo - to bear witness) means that God corroborated their spoken testimony. It is as if Almighty God placed what they said in bold letters, italicized and followed with an exclamation point!

So we see in this exhortational interlude:

Character of Christ
Certainty of Judgment
Confirmation of God.

When Jesus preached the gospel, He also did some things that made it even more believable. He said,

Though you do not believe Me, believe the works, that you may know and understand that the Father is in Me, and I in the Father (Jn 10:38).

When He claimed to be God and then did things that only God could do, He confirmed His divinity and, consequently, the truth of His message. On the Day of Pentecost Peter reminded his hearers that

Jesus the Nazarene [was] a man attested to you by God with miracles and wonders and signs (Acts 2:22)

God gave similar confirming signs through the apostles, the first preachers of the gospel after Christ Himself. Many of their listeners no doubt said, “Why should we believe them? What proof do we have that their message is from God? There have always been false teachers. How can we know that these are true?” So God bore His apostles witness by giving them the ability to do what Jesus had done - signs, wonders, and miracles.

BOTH BY SIGNS AND WONDERS AND BY VARIOUS MIRACLES: semeiois te kai terasin kai poikilais dunamesin:

Both (5037) (kai) in general is used to couple ideas which follow directly and necessarily from what precedes, while the "te" is employed generally when something is subjoined which does not thus directly and necessarily follow. Kaí connects and te annexes. Hence, te is the most general of all the copulatives (serving merely to show that the word or words preceding it has some connection with the one or ones following it). The place of te is usually after the first word of a clause.

Signs (4592) (semeion from sema = sign or mark) describes an event which is regarded as having some special meaning. Semeion is that by which something is known or distinguished. The related verb semaino means to cause something to be both specific and clear. The root word sema, was used for example by Homer to describe optical impressions that convey insights, e.g., signs like lightning that indicate the will of Zeus. Thus the signs may be simply pointers and as such are characterized by prominence and visibility.

TDNT observes that signs refer to

The general sense of a mark by which someone or something is recognized makes possible a varied use, e.g., for monuments, finishing posts in races, or identifying marks on the body.  Despite divergent use, the sense is uniform. What is meant is an object or circumstance that conveys a perception or insight. The perception may be moral or religious, but the term as such is not intrinsically a religious one." (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

See Acts 2:22 for the three words for miracles in inverse order (powers, wonders, signs).

Signs speak of the "fingerprints" of God, valuable not so much for what they are as for what they indicate of the grace and power of the Almighty. The use of the Greek word semeion indicates that the event is not an empty show of power, but significantly points to the reality of the mighty hand of God in operation. 

Semeion may or may not be supernatural like the dunamis and teras. They are "signs" in the sense of being comparable to prophecies or spiritual truths. Semeion stresses the spiritual truth embodied in the miracle (Jn 20:30, 31).

Wonders (5059) (teras from tereo = to keep, watch)  emphasizes that which due to its extraordinary character is apt to be observed and kept in the memory. It is a miracle regarded as startling, imposing or amazing.

Signs emphasizes the divine communication in the miraculous.

Wonders stresses impression made upon those witnessing the miracle; compel one's attention or cause one to "look again".

Miracles is better rendered "powers" and calls attention to the awesome abilities which produce the miraculous.

Signs reveal the purpose of God in the miracles.

Wonders attract attention.

Miracles show God's power.

The greatest sign, wonder and miracle is when bad men are made holy and righteous by God's grace through faith as testified by changed lives. Truly miracles in Christianity are still evident today for all to behold. People are hungry to see the power of Christ in reality, not the charade of charlatans masquerading as spiritual magicians or carnival side show freaks.

Various (4164) (poikilos) means existence in various kinds or modes, diversified, manifold, variegated, many colored. Poikilos was used to describe the skin of a leopard, the different-colored veining of marble or an embroidered robe and thence passes into the meaning of changeful, diversified, applied to the changing months or the variations of a strain of music.

Miracles (1411) (dunamis from dunamai = to be able, to have power) speaks of power and especially of achieving power. The chief idea is that of something with intrinsic power or inherent ability, the power or ability to carry out some function, the potential for functioning in some way (power, might, strength, ability, capability), the power residing in a thing by virtue of its nature. Dunamis is the implied ability or capacity to perform. It conveys the idea of effective, productive energy, rather than that which is raw and unbridled. Dunamis is the word generally used by Paul of divine energy. The writer of Hebrews uses dunamis to describe deeds that exhibit the ability to function powerfully -- deeds of power or miracles. These miracles attested the spoken word of the apostles before it was given in written form

Paul writes that...

The signs of a true apostle were performed among you with all perseverance, by signs and wonders and miracles. (2 Corinthians 12:12)

AND BY GIFTS OF THE HOLY SPIRIT: kai pneumatos hagiou merismois:

Gifts (3311) (merismos from merizo = to divide into parts) refers to the act of distribution, separation, or that which is distributed and is used only here and in Hebrews 4:12 where it is translated "piercing as far as the division (merismos) of soul and spirit, of both joints and marrow..."

See Spiritual Gifts Chart and also resources on Spiritual Gifts

The Holy Spirit's role in giving of spiritual gifts is described in 1 Corinthians 12:

4 Now there are varieties of gifts, but the same Spirit. 5 And there are varieties of ministries, and the same Lord. 6 And there are varieties of effects, but the same God who works all things in all persons. 7 But to each one is given the manifestation of the Spirit for the common good. 8 For to one is given the word of wisdom through the Spirit, and to another the word of knowledge according to the same Spirit; 9 to another faith by the same Spirit, and to another gifts of healing by the one Spirit, 10 and to another the effecting of miracles, and to another prophecy, and to another the distinguishing of spirits, to another various kinds of tongues, and to another the interpretation of tongues. 11 But one and the same Spirit works all these things, distributing to each one individually just as He wills." (1 Corinthians 12:4-11)

In Ephesians Paul records that...

"WHEN HE ASCENDED ON HIGH, HE LED CAPTIVE A HOST OF CAPTIVES, AND HE GAVE GIFTS TO MEN." 9 (Now this expression, "He ascended," what does it mean except that He also had descended into the lower parts of the earth? 10 He who descended is Himself also He who ascended far above all the heavens, that He might fill all things.) 11 And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers, 12 for the equipping of the saints for the work of service, to the building up of the body of Christ; 13 until we all attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fulness of Christ. 14 As a result, we are no longer to be children, tossed here and there by waves, and carried about by every wind of doctrine, by the trickery of men, by craftiness in deceitful scheming" (See notes Ephesians 4:8; 4:9; 4:10; 4:11)

ACCORDING TO HIS OWN WILL: kata ten autou thelesin: (Daniel 4:35; Ephesians 1:5,9)

According to the His own will - This is the qualifying phrase regarding the gifts. They are not capriciously given or at random but are divinely in accord with the will of God.

A similar statement is made by Paul in 1 Corinthians 12:11...

But one and the same Spirit works all these things, distributing to each one individually just as He wills.

In other words the Holy Spirit distributed the gifts to each one individually (thus every believer has at least one gift) and just as He wills.

Will (2308) (thelesis) refers to the act of willing as contrasted to thelema which objectively is that which is willed. Thelesis emphasizes the active exercise of will. God's free and sovereign will, assigning one gift of the Spirit to one, another to another as He chose. In other words God as the Sovereign Ruler of all creation gave where he pleased, and imparted in such measure as He chose.